Conventional U-Pb dating versus SHRIMP of the Santa Barbara Granite Massif, Rondonia, Brazil

USGS Publications Warehouse

Sparrenberger, I.; Bettencourt, Jorge S.; Tosdal, R.M.; Wooden, J.L.

2002-01-01

The Santa Ba??rbara Granite Massif is part of the Younger Granites of Rondo??nia (998 - 974 Ma) and is included in the Rondo??nia Tin Province (SW Amazonian Craton). It comprises three highly fractionated metaluminous to peraluminous within-plate A-type granite units emplaced in older medium-grade metamorphic rocks. Sn-mineralization is closely associated with the late-stage unit. U-Pb monazite conventional dating of the early-stage Serra do Cicero facies and late-stage Serra Azul facies yielded ages of 993 ?? 5 Ma and 989 ?? 13 Ma, respectively. Conventional multigrain U-Pb isotope analyses of zircon demonstrate isotopic disturbance (discordance) and the preservation of inherited older zircons of several different ages and thus yield little about the ages of Sn-granite magmatism. SHRIMP U-Pb ages for the Santa Ba??rbara facies association yielded a 207Pb/206Pb weighted-mean age of 978 ?? 13 Ma. The textural complexity of the zircon crystals of the Santa Ba??rbara facies association, the variable concentrations of U, Th and Pb, as well as the mixed inheritance of zircon populations are major obstacles to using conventional multigrain U-Pb isotopic analyses. Sm-Nd model ages and ??Nd (T) values reveal anomalous isotopic data, attesting to the complex isotopic behaviour within these highly fractionated granites. Thus, SHRIMP U-Pb zircon and conventional U-Pb monazite dating methods are the most appropriate to constrain the crystallization age of the Sn-bearing granite systems in the Rondo??nia Tin Province.

Chemical abrasion-SIMS (CA-SIMS) U-Pb dating of zircon from the late Eocene Caetano caldera, Nevada

USGS Publications Warehouse

Watts, Kathryn E.; Coble, Matthew A.; Vazquez, Jorge A.; Henry, Christopher D.; Colgan, Joseph P.; John, David A.

2016-01-01

Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206Pb/238U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6 Myr older than the 40Ar/39Ar sanidine eruption age (34.00 ± 0.03 Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3 Myr younger. U-Pb ages do not correlate with U (~ 100–800 ppm), Th (~ 50–300 ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~ 1.6 μm for CA and non-CA Caetano, R33 and TEMORA-2 zircons, and do not indicate variations in secondary ion sputtering rates due to chemical or structural changes from the CA treatment. Our new data underscore the potential for cryptic Pb-loss to go unrecognized in other geologically young magmatic centers that do not have zircons with high U, statistically discordant isotope ratios, high common Pb, or metamict textures.

U-Pb SHRIMP dating of uraniferous opals

USGS Publications Warehouse

Nemchin, A.A.; Neymark, L.A.; Simons, S.L.

2006-01-01

U-Pb and U-series analyses of four U-rich opal samples using sensitive high-resolution ion microprobe (SHRIMP) demonstrate the potential of this technique for the dating of opals with ages ranging from several tens of thousand years to millions of years. The major advantages of the technique, compared to the conventional thermal ionisation mass spectrometry (TIMS), are the high spatial resolution (???20 ??m), the ability to analyse in situ all isotopes required to determine both U-Pb and U-series ages, and a relatively short analysis time which allows obtaining a growth rate of opal as a result of a single SHRIMP session. There are two major limitations to this method, determined by both current level of development of ion probes and understanding of ion sputtering processes. First, sufficient secondary ion beam intensities can only be obtained for opal samples with U concentrations in excess of ???20 ??g/g. However, this restriction still permits dating of a large variety of opals. Second, U-Pb ratios in all analyses drifted with time and were only weakly correlated with changes in other ratios (such as U/UO). This drift, which is difficult to correct for, remains the main factor currently limiting the precision and accuracy of the U-Pb SHRIMP opal ages. Nevertheless, an assumption of similar behaviour of standard and unknown opals under similar analytical conditions allowed successful determination of ages with precisions of ???10% for the samples investigated in this study. SHRIMP-based U-series and U-Pb ages are consistent with TIMS dating results of the same materials and known geological timeframes. ?? 2005 Elsevier B.V. All rights reserved.

Parent zonation in thermochronometers - resolving complexity revealed by ID-TIMS U-Pb dates and implications for the application of decay-based thermochronometers

NASA Astrophysics Data System (ADS)

Navin Paul, Andre; Spikings, Richard; Chew, David; Daly, J. Stephen; Ulyanov, Alexey

2017-04-01

High temperature (>350℃) U-Pb thermochronometers primarily use accessory minerals such as apatite, titanite and rutile, and assume that daughter isotopes are lost by thermally activated volume diffusion while the parent remains immobile. Studies exploiting such behaviour have been successfully used to reconstruct thermal histories spanning several hundred million years (e.g. Cochrane et al., 2014). However, outliers in date (ID-TIMS) vs diffusion length space are frequently observed, and grains are frequently found to be either too young or too old for expected thermal history solutions using the diffusion data of Cherniak et al. (2010). These deviations of single grain apatite U-Pb dates from expected behaviour could be caused by a combination of i) metamorphic (over-)growth, ii) fluid-aided Pb mobilisation during alteration/recrystallization, iii) parent isotope zonation, iv) metamictisation, and v) changes in diffusion length with time (e.g. fracturing). We present a large data set from the northern Andes of South America, where we compare apatite U-Pb ID-TIMS-(TEA) data with LA-ICP-MS element maps and in-situ apatite U-Pb LA-(MC)-ICP-MS dates. These are combined with U-Pb zircon and 40Ar/39Ar (muscovite) data to attempt to distinguish between thermally activated volume diffusion and secondary overgrowth/recrystallization. We demonstrate that in young (e.g. Phanerozoic) apatites that have not recrystallized or experienced metasomatic overgrowths, U-Pb dates are dominantly controlled by volume diffusion and intra-crystal uranium zonation. This implies that ID-TIMS analyses of apatites with zoned parent isotope distributions will not usually recover accurate thermal history solutions, and an in-situ dating method is required. Recovering the uranium distribution during in-situ analysis provides a means to account for parent zonation, substantially increasing the accuracy of the modelled t-T-paths. We present in-situ data from apatites where scatter in date v diffusion length scale is observed and compare t-T-paths from single grain and in-situ modelling. Modelling of in-situ data will further show if all apatites from a single hand specimen record the same thermal history using Cherniak et al. (2010) diffusion data, or if the Pb-in-apatite diffusion parameters are a function of composition. U zonation is ubiquitous in the studied rocks (Triassic apatites extracted from peraluminous leucosomes), implying that these conclusions may also apply to lower temperature thermochronometers that are based on uranium decay, such as (U-Th)/He dating.

Integrated Laser Ablation U/Pb and (U-Th)/He Dating of Detrital Accessory Minerals from the Naryani River, Central Nepal

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2015-12-01

The newly developed 'laser ablation double dating' (LADD) technique, an integrated laser microprobe U/Pb and (U-Th)/He dating method, could be an exceptionally valuable tool in detrital thermochronology for identifying sedimentary provenance and evaluating the exhumation history of a source region. A recent proof-of-concept study has used LADD to successfully date both zircon and titanite crystals from the well-characterized Fish Canyon tuff, but we also believe that another accessory mineral, rutile, could be amenable to dating via the LADD technique. To continue the development of the method, we present an application of LADD to detrital zircon, titanite, and rutile from a sample collected on the lower Naryani River of central Nepal. Preliminary analyses of the sample have yielded zircon U/Pb dates ranging from 31.4 to 2405 Ma; zircon (U-Th)/He from 1.8 to 15.4 Ma; titanite U/Pb between 18 and 110 Ma; titanite (U-Th)/He between 1 and 16 Ma; rutile U/Pb from 6 to 45 Ma; and rutile (U-Th)/He from 2 to 25 Ma. In addition to the initial data, we can use Ti-in-zircon, Zr-in-titanite, and Zr-in-rutile thermometers to determine the range of possible long-term cooling rates from grains with U/Pb ages younger than collision. Thus far our results from zircon analyses imply a cooling rate of approximately 15°C/Myr; titanite analyses imply between 10 and 67°C/Myr; and rutile between 9 and 267°C/Myr. This spread in potential cooling rates, especially in the order of magnitude differences of cooling rates calculated from the rutile grains, suggests that the hinterland source regions of the Naryani river experienced dramatically different exhumation histories during Himalayan orogenisis. Ongoing analyses will expand the dataset such that we can more adequately characterize the range of possibilities represented in the sample.

Combined garnet and zircon geochronology of the ultra-high temperature metamorphism: Constraints on the rise of the Orlica-Śnieżnik Dome, NE Bohemian Massif, SW Poland

NASA Astrophysics Data System (ADS)

Walczak, Katarzyna; Anczkiewicz, Robert; Szczepański, Jacek; Rubatto, Daniela; Košler, Jan

2017-11-01

Garnet and zircon geochronology combined with trace element partitioning and petrological studies provide tight constraints on evolution of the UHT-(U)HP terrain of the Orlica-Śnieżnik Dome (OSD) in the NE Bohemian massif. Lu-Hf dating of peritectic garnet from two mesocratic granulites constrained the time of its initial growth at 346.9 ± 1.2 and 348.3 ± 2.0 Ma recording peak 2.5 GPa pressure and 950 °C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9 ± 3.4 Ma. Ti-in-zircon thermometry indicates crystallization at 810-860 °C pointing to zircon formation on the retrograde path. Lu partitioning between garnet rim and zircon suggest equilibrium growth and thus U-Pb zircon age constrain the terminal phase of garnet crystallization which lasted about 6 Ma. All Sm-Nd garnet ages obtained for mesocratic and mafic granulites are identical and consistently younger than the corresponding Lu-Hf dates. They are interpreted as reflecting cooling of granulites through the Sm-Nd closure temperature at about 337 Ma. The estimated PTt path documents the ca. 10 Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347 - > 342 Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at > 342-337 Ma which corresponds to a fast, nearly isobaric cooling.

New Robust Reference Materials for In Situ Single Grain Rutile U-Pb Geochronology and Method Refinements for Detrital Rutile Analysis by LA-MC-ICP-MS

NASA Astrophysics Data System (ADS)

Parrish, R. R.; Bracciali, L.; Condon, D. J.; Horstwood, M. S.; Najman, Y.

2012-12-01

While rutile (TiO2) occurs in the heavy mineral suite of detrital sediments and originates mainly in medium- to high-grade metamorphic and some igneous rocks, there are very few applications of U-Pb dating of rutile to provenance studies; this is due to an overreliance on zircon, low U content of rutile limiting measurement quality by in situ methods, a higher proportion of common Pb relative to zircon, and a lack of widely available good quality reference materials. We have addressed these issues and characterized two ~ 1.8 Ga rutile reference materials by SEM, trace elements, U-Pb ID-TIMS, and intra-grain and inter-grain U-Pb LA-MC-ICP-MS analysis using mixed faraday and multiple ion counting detectors with high sensitivity. We have assessed U-Pb discordance and in situ variations in relative common Pb and age and their bearing on the quality of the reference materials for in situ U-Pb dating. The rutiles (Sugluk-4 and PCA-S207) come from granulite facies belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). The ID-TIMS data are slightly discordant due to variable common Pb and limited Pb loss; the variation in 6 single grains of Sugluk-4, that we use as the primary reference material, is <1% in 206Pb/238U, and <2% for 207Pb/206Pb (95 % conf.); after common Pb correction these variations are <1%. The measured variations are smaller than in existing reference materials (i.e. R10) in current use. LA-ICP-MC-MS data (n ~ 500 for each) have a reproducibility of 206Pb/238U and 207Pb/206Pb of ~2-4% (at the 2S level), which is only modestly worse than long-term data for multiple zircon standards, this being due to the real variation in measured values arising from limited Pb loss, age variation, and common Pb variability [1]. We have applied our refined method to the provenance of rutile from drainages from British Columbia, Bhutan, and the Brahmaputra River of NE India (predominant rutile ages ~ 50, 15, and 2 Ma, respectively; Bracciali et al., this meeting). Our method successfully dates >75% of all rutile grains in a sediment; unsuccessful analyses are due to poor quality rutiles with massive common Pb and/or U contents < ~1ppm. While some analyses are therefore unusable, unlike zircon age zoning is rare to absent in rutile and there is little need to image grains to identify 'inheritance' to arrive at a correct interpretation of measured ages. Rutile has a ~ 500°C closure temperature and thus records mainly the time of cooling; it is therefore a sensitive recorder of metamorphic thermochronological information, and an excellent complement to detrital zircon analysis. There appears to be huge scope of in situ application of U-Pb dating to detrital rutile in provenance studies in the future. [1] Bracciali L., Parrish R.R., Condon D., Horstwood M.S.A., Najman,Y., Two new rutile reference materials for in situ U-Pb LA-MC-ICP-MS dating and applications to sedimentary provenance, submitted to Chem. Geol.

Constraining Metamorphic Timing and Processes by Dating Garnet, Zircon, Titanite and Monazite in UHP and HP Rocks from Weihai, Sulu UHP Terrane, Eastern China

NASA Astrophysics Data System (ADS)

Wang, D.; Vervoort, J. D.; Fisher, C. M.; Cao, H.

2016-12-01

The Sulu UHP terrane is the extension of the Dabie orogenic belt to the east, offset 500 km to the northeast by the Tanlu fault [1]. The focus of this study, the Weihai area, is located at the northernmost part of the Sulu UHP terrane, and consists mainly of gneisses overprinted by amphibolite-facies assemblages, in addition to minor eclogite, granulite, and some ultramafic rocks [1]. Time constrains are critical to our understanding of the processes of UHP metamorphism, as well as the tectonic evolution of the region. In the last decade, U-Pb dating of metamorphic domains of zircons has been widely applied to determine the history of the UHP metamorphism (240 - 220 Ma) [1]. Recent garnet Lu-Hf dating from the Dabie terrane (240 - 220Ma) suggests the initiation of prograde metamorphism to be prior to ca. 240 Ma [2]. In-situ U-Pb dating of accessary minerals using LA-ICPMS (i.e. monazite, titanite, rutile, etc.), can provide important information to augment and complement the zircon U-Pb metamorphic dates. In this study, we collected samples throughout the Weihai area. Protolith ages of these samples range from Paleoproterozoic to Neoproterozoic ( 1850 - 700 Ma) as indicated by U-Pb dating of zircon cores. Zircon metamorphic rims yield U-Pb ages of 240 - 220 Ma, likely indicating the UHP stage of the Sulu terrane [3]. Four eclogites yield Lu-Hf garnet isochrons with dates between 239 and 224 Ma, consistent with garnet Lu-Hf dates from Dabie UHP terrane [2]. Sm-Nd isochrons indicate systematic younger dates (220 - 210 Ma) interpreted as cooling ages. Titanites extracted from four samples give U-Pb ages ranging from 220 to 200 Ma, in agreement with the titanite dates from the southern Sulu terrane [4]. Monazites from three samples give precise dates between 214 and 211 Ma. Collectively, monazite and titanite U-Pb ages are broadly consistent with the garnet Sm-Nd isochrons, and thus we interpret these as cooling ages. Based on the dates of different systems/minerals presented above, we suggest the prograde metamorphism of Weihai UHP terrane likely took place prior to 240 Ma, and the peak of the UHP stage is likely between 240 and 220 Ma. [1] Zhang et al., Gondwana Res., 16 (2009) 1-26 [2] Cheng et al., J. Metamorphic Geol., 26 (2008), 741-758 [3] Liou et al., J. Asian Earth Sci., 35 (2009), 199-231 [4] Chen and Zheng, GCA, 150(2015), 53-73

Enhanced provenance interpretation using combined U-Pb and (U-Th)/He double dating of detrital zircon grains from lower Miocene strata, proximal Gulf of Mexico Basin, North America

NASA Astrophysics Data System (ADS)

Xu, Jie; Stockli, Daniel F.; Snedden, John W.

2017-10-01

Detrital zircon U-Pb analysis is an effective approach for investigating sediment provenance by relating crystallization age to potential crystalline source terranes. Studies of large passive margin basins, such as the Gulf of Mexico Basin, that have received sediment from multiple terranes with non-unique crystallization ages or sedimentary strata, benefit from additional constraints to better elucidate provenance interpretation. In this study, U-Pb and (U-Th)/He double dating analyses on single zircons from the lower Miocene sandstones in the northern Gulf of Mexico Basin reveal a detailed history of sediment source evolution. U-Pb age data indicate that most zircon originated from five major crystalline provinces, including the Western Cordillera Arc (<250 Ma), the Appalachian-Ouachita orogen (500-260 Ma), the Grenville (1300-950 Ma) orogen, the Mid-Continent Granite-Rhyolite (1500-1300 Ma), and the Yavapai-Mazatzal (1800-1600 Ma) terranes as well as sparse Pan-African (700-500 Ma) and Canadian Shield (>1800 Ma) terranes. Zircon (U-Th)/He ages record tectonic cooling and exhumation in the U.S. since the Mesoproterozoic related to the Grenville to Laramide Orogenies. The combined crystallization and cooling information from single zircon double dating can differentiate volcanic and plutonic zircons. Importantly, the U-Pb-He double dating approach allows for the differentiation between multiple possible crystallization-age sources on the basis of their subsequent tectonic evolution. In particular, for Grenville zircons that are present in all of lower Miocene samples, four distinct zircon U-Pb-He age combinations are recognizable that can be traced back to four different possible sources. The integrated U-Pb and (U-Th)/He data eliminate some ambiguities and improves the provenance interpretation for the lower Miocene strata in the northern Gulf of Mexico Basin and illustrate the applicability of this approach for other large-scale basins to reconstruct sediment provenance and dispersal patterns.

Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico: COMMENT

USGS Publications Warehouse

Koenig, Alan E.; Lucas, Spencer G.; Neymark, Leonid A.; Heckert, Andrew B.; Sullivan, Robert M.; Jasinski, Steven E.; Fowler, Denver W.

2012-01-01

Based on U-Pb dating of two dinosaur bones from the San Juan Basin of New Mexico (United States), Fassett et al. (2011) claim to provide the first successful direct dating of fossil bones and to establish the presence of Paleocene dinosaurs. Fassett et al. ignore previously published work that directly questions their stratigraphic interpretations (Lucas et al., 2009), and fail to provide sufficient descriptions of instrumental, geochronological, and statistical treatments of the data to allow evaluation of the potentially complex diagenetic and recrystallization history of bone. These shortcomings lead us to question the validity of the U-Pb dates published by Fassett et al. and their conclusions regarding the existence of Paleocene dinosaurs.

Heterogeneity in the 238U/235U Ratios of Angrites.

NASA Astrophysics Data System (ADS)

Tissot, F.; Dauphas, N.; Grove, T. L.

2016-12-01

Angrites are differentiated meteorites of basaltic composition, of either volcanic or plutonic origin, that display minimal post-crystallization alteration, metamorphism, shock or impact brecciation. Because quenched angrites cooled very rapidly, all radiochronometric systems closed simultaneously in these samples. Quenched angrites are thus often used as anchors for cross-calibrating short-lived dating methods (e.g., 26Al-26Mg) and the absolute dating techniques (e.g, Pb-Pb). Due to the constancy of the 238U/235U ratio in natural samples, Pb-Pb ages have long been calculated using a "consensus" 238U/235U ratio, but the discovery of resolvable variations in the 238U/235U ratio of natural samples, means that the U isotopic composition of the material to date also has to be determined in order to obtain high-precision Pb-Pb ages. We set out (a) to measure at high-precision the 238U/235U ratio of a large array of angrites to correct their Pb-Pb ages, and (b) to identify whether all angrites have a similar U isotopic composition, and, if not, what were the processes responsible for this variability. Recently, Brennecka & Wadhwa (2012) suggested that the angrite-parent body had a homogeneous 238U/235U ratio. They reached this conclusion partly because they propagated the uncertainties of the U isotopic composition of the various U double spikes that they used onto the final 238U/235U ratio the sample. Because this error is systematic (i.e., it affects all samples similarly), differences in the δ238U values of samples corrected by the same double spike are better known than one would be led to believe if uncertainties on the spike composition are propagated. At the conference, we will present the results of the high-precision U isotope analyses for six angrite samples: NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D'Orbigny, and Sahara 99555. We will show that there is some heterogeneity in the δ238U values of the angrites and will discuss the possible processes by which different angrite samples can acquire different U isotopic compositions. The U isotope data will then be used to correct Pb-Pb ages of angrites estimated using an assumed 238U/235U ratio. These ages will be used to discuss the degree of concordance between short-lived nuclides systems and the absolute Pb-Pb clock in early Solar System materials.

Petrogenesis of the Yaochong granite and Mo deposit, Western Dabie orogen, eastern-central China: Constraints from zircon U-Pb and molybdenite Re-Os ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes

NASA Astrophysics Data System (ADS)

Chen, Wei; Xu, Zhaowen; Qiu, Wenhong; Li, Chao; Yu, Yang; Wang, Hao; Su, Yang

2015-05-01

The Dabie orogen is among the most famous continent-continent collisional orogenic belts in the world, and is characterized by intensive post-collisional extension, magmatism and Mo mineralization. However, the genetic links between the mineralization and the geodynamic evolution of the orogen remain unresolved. In this paper, the Yaochong Mo deposit and its associated granitic stocks were investigated to elucidate this issue. Our new zircon U-Pb ages yielded an Early Cretaceous age (133.3 ± 1.3 Ma) for the Yaochong granite, and our molybdenite Re-Os dating gave a similar age (135 ± 1 Ma) for the Mo deposit. The Yaochong stock is characterized by high silica and alkali but low Mg, Fe and Ca. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: Rb, K, Th and U), but strongly depleted in heavy REEs, and high field strength elements (HFSEs: Nb, Ta, Ti and Y). The Yaochong granite has initial 87Sr/86Sr ratios of 0.7087-0.7096, and Pb isotopic ratios of (206Pb/204Pb)i = 16.599-16.704, (207Pb/204Pb)i = 15.170-15.618 and (208Pb/204Pb)i = 36.376-38.248. The granite has εNd(t) of -18.0 to -16.3 and εHf(t) values of -26.5 to -20.0. All these data indicate that the Yaochong granite is a high-K calc-alkaline fractionated I-type granite, and may have originated from partial melting of the thickened Yangtze continental crust. The Mo ores also show low radiogenic Pb isotopes similar to the Yaochong stock. Medium Re content in molybdenite (21.8-74.8 ppm) also suggests that the ore-forming materials were derived from the thickened lower crust with possibly minor mixing with the mantle. Similar to the Eastern Dabie orogen, the thickened crust beneath the Western Dabie orogen may also have experienced tectonic collapse, which may have exerted fundamental geodynamic controls on the two-stage Mo mineralization in the region.

Finding the "true" age: ways to read high-precision U-Pb zircon dates

NASA Astrophysics Data System (ADS)

Schaltegger, U.; Schoene, B.; Ovtcharova, M.; Sell, B. K.; Broderick, C. A.; Wotzlaw, J.

2011-12-01

Refined U-Pb dating techniques, applying an empirical chemical abrasion treatment prior to analysis [1], and using a precisely calibrated double isotope Pb, U EARTHTIME tracer solution, have led to an unprecedented <0.1% precision and accuracy of obtained 206Pb/238U dates of single zircon crystals or fragments. Results very often range over 10e4 to 10e6 years and cannot be treated as statistically singular age populations. The interpretation of precise zircon U-Pb ages is biased by two problems: (A) Post-crystallization Pb loss from decay damaged areas is considered to be mitigated by applying chemical abrasion techniques. The success of such treatment can, however, not be assumed a priori. The following examples demonstrate that youngest zircons are not biased by lead loss but represent close-to-youngest zircon growth: (i) coincidence of youngest zircon dates with co-magmatic titanite in tonalite; (ii) coincidence with statistically equivalent clusters of 206Pb/238U dates from zircon in residual melts of cogenetic mafic magmas; (iii) youngest zircons in ash beds of sedimentary sequences do not violate the stratigraphic superposition, whereas conventional statistical interpretation (mean or median values) does; (iv) results of published inter-laboratory cross-calibration tests using chemical abrasion on natural zircon crystals of the same sample arrive at the same 206Pb/238U result within <0.1% (e.g., [2]); (v) Youngest crystals coincide in age with the astronomical age of hosting cyclic sediments. Residual lead loss may, however, still be identified in the case of single, significantly younger dates (>3 sigma), and are common in many pre-Triassic and hydrothermally altered rocks. (B) Pre-eruptive/pre-intrusive growth is found to be the main reason for scattered zircon ages in igneous rocks. Zircons crystallizing from the final magma batch are called autocrystic [3]. Autocrystic growth will happen in a moving or stagnant magma shortly before or after the rheological lockup by the crystals. Last crystallizing zircons in the interstitial melt may therefore postdate emplacement of the magma. The range of 206Pb/238U ages may yield a time frame for the cooling of a given magma batch, which could be added to quantitative thermal models of magma emplacement and cooling. Hf isotopes and trace elements of the dated zircon are used to trace the nature of the dated grains [4], specifically for identification of crystals that form earlier at lower crustal levels (antecrysts). Autocrystic zircons typically show, e.g., distinctly different (higher or lower) Th/U ratios. Cautiously interpreted high-precision U-Pb data of chemically abraded zircons may resolve the evolution of a magmatic system from its roots to final emplacement or eruption, trace fractional crystallization of zircon and other accessory and major phases in a magma batch, and add quantitative temporal constraints to thermal models. The proposed interpretation scheme thus adds significant information compared to conventional statistics. [1] Mattinson J., 2005, Chem. Geol. 200, 47-66; ; [2] Slama et al., 2008, Chem. Geol. 249, 1-35; [3] Miller et al., 2007, J. Volc. Geotherm. Res. 167, 282-299; [4] Schoene et al., 2010, Geochim. Cosmochim. Acta 74, 7144-7159

Combined apatite fission track and U-Pb dating by LA-ICPMS

NASA Astrophysics Data System (ADS)

Chew, D. M.; Donelick, R. A.

2012-04-01

Apatite is a common accessory mineral in igneous, metamorphic and clastic sedimentary rocks. It is a nearly ubiquitous accessory phase in igneous rocks, is common in metamorphic rocks of pelitic, carbonate, basaltic, and ultramafic composition and is virtually ubiquitous in clastic sedimentary rocks. In contrast to the polycyclic behavior of the stable heavy mineral zircon, apatite is unstable in acidic groundwaters and has limited mechanical stability in sedimentary transport systems. Apatite has many potential applications in provenance studies, particularly as it likely represents first-cycle detritus. Fission track and U-Pb dating are very powerful techniques in apatite provenance studies. They yield complementary information, with the apatite fission-track system yielding low-temperature exhumation ages and the U-Pb system yielding high-temperature cooling ages which constrain the timing of apatite crystallization. This study focuses on integrating apatite fission track and U-Pb dating by the LA-ICPMS method. Our approach is intentionally broad in scope, and is applicable to any quadrupole or rapid-scanning magnetic-sector LA-ICPMS system. Calculating uranium concentrations in fission-track dating by LA-ICPMS increases the speed of analysis and sample throughput compared to the conventional external detector method and avoids the need for neutron irradiation (Hasebe et al., 2004). LA-ICPMS-based uranium measurements in apatite are measured relative to an internal concentration standard (typically 43Ca). Ca in apatite is not always stochiometric as minor cations (Mn2+, Sr2+, Ba2+ and Fe2+) and REE can substitute with Ca2+. These substitutions must be quantified by multi-elemental LA-ICPMS analyses. Such data are also useful for discriminating between different apatite populations in sedimentary or volcaniclastic rocks based on their trace-element chemistry. Low U, Th and radiogenic Pb concentrations, elevated common Pb / radiogenic Pb ratios and U-Pb elemental fractionation are challenges in apatite U-Pb dating by LA-ICPMS. Isochron-based approaches to common Pb correction require a significant spread in common Pb / radiogenic Pb ratios. This is not usually possible on individual detrital apatite grains and hence the 204Pb-, 207Pb- and 208Pb-correction methods are preferred. Uranium concentration measurements by ICPMS employ large peak jumps (the internal standard is a Ca isotope) which require a quadrupole or a rapid-scanning magnetic-sector LA-ICPMS system. These single-collector instruments require a prohibitively long dwell time on the low intensity 204Pb peak to measure it accurately and hence the 207Pb- and 208Pb-correction methods are preferred. Uranium-concentration measurements in fission-track dating require well-constrained ablation depths during analysis and hence spot analyses are preferred to rastering. Laser-induced U-Pb fractionation is corrected for by sample-standard bracketing using a variety of apatite standards (Durango, Emerald Lake, Fish Canyon Tuff, Kovdor, Otter Lake and McClure Mountain syenite). Of these, Emerald Lake (Chew et al., 2011) and McClure Mountain syenite apatite are recommended as primary standards with Durango apatite making a suitable secondary standard. Offline data-reduction uses custom-written software for ICPMS data processing (the UPbICP package of Ray Donelick) or the freeware IOLITE data-reduction package of Paton et al. (2010).

Modern U-Pb chronometry of meteorites: advancing to higher time resolution reveals new problems

USGS Publications Warehouse

Amelin, Y.; Connelly, J.; Zartman, R.E.; Chen, J.-H.; Gopel, C.; Neymark, L.A.

2009-01-01

In this paper, we evaluate the factors that influence the accuracy of lead (Pb)-isotopic ages of meteorites, and may possibly be responsible for inconsistencies between Pb-isotopic and extinct nuclide timescales of the early Solar System: instrumental mass fractionation and other possible analytical sources of error, presence of more than one component of non-radiogenic Pb, migration of ancient radiogenic Pb by diffusion and other mechanisms, possible heterogeneity of the isotopic composition of uranium (U), uncertainties in the decay constants of uranium isotopes, possible presence of "freshly synthesized" actinides with short half-life (e.g. 234U) in the early Solar System, possible initial disequilibrium in the uranium decay chains, and potential fractionation of radiogenic Pb isotopes and U isotopes caused by alpha-recoil and subsequent laboratory treatment. We review the use of 232Th/238U values to assist in making accurate interpretations of the U-Pb ages of meteorite components. We discuss recently published U-Pb dates of calcium-aluminum-rich inclusions (CAIs), and their apparent disagreement with the extinct nuclide dates, in the context of capability and common pitfalls in modern meteorite chronology. Finally, we discuss the requirements of meteorites that are intended to be used as the reference points in building a consistent time scale of the early Solar System, based on the combined use of the U-Pb system and extinct nuclide chronometers.

Precise U-Pb Zircon Constraints on the Earliest Magmatic History of the Carolina Terrane.

PubMed

Wortman; Samson; Hibbard

2000-05-01

The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum-Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U-Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow-banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U-Pb zircon date of 632.9 +2.6/-1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U-Pb zircon date of 633 +2/-1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U-Pb zircon dates of 619.9 +4.5/-3 Ma and 615.7 +3.7/-1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U-Pb upper-intercept dates of 613.9 +1.6/-1.5 Ma and 613.4 +2.8/-2 Ma. The Osmond biotite-granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/-1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U-Pb upper intercept date of 546.5 +3.0/-2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/-1.5 Ma and continued at least until 612.4 +5.2/-1.7 Ma, an interval of approximately 25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/-1.7 Ma and 586+/-10 Ma (reported age of the upper Uwharrie Formation). The U-Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental-type basement. This observation, together with the juvenile Nd isotopic character of the Virgilina volcanic arc sequence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence.

Marine and Lacustrine Organic-rich Sedimentary Unit Time Markers: Implications from Rhenium-Osmium Geochronology

NASA Astrophysics Data System (ADS)

Selby, D.

2011-12-01

Geochronology is fundamental to understand the age, rates and durations of Earth processes. This concerned Arthur Holmes who, for much of his career, attempted to define a geological time scale. This is a topic still important to Earth Scientists today, specifically the chronostratigraphy of sedimentary rocks. Here I explore the Re-Os geochronology of marine and lacustrine sedimentary rocks and its application to yield absolute time constraints for stratigraphy. The past decade has seen the pioneering research of Re-Os organic-rich sedimentary rock geochronology blossom into a tool that can now to be used to accurately and precisely determine depositional ages of organic-rich rock units that have experienced up to low grade greenschist metamorphism. This direct dating of sedimentary rocks is critical where volcanic horizons are absent. As a result, this tool has been applied to timescale calibration, basin correlation, formation duration and the timing of key Earth events (e.g., Neoproterozoic glaciations). The application of Re-Os chronometer to the Devonian-Mississippian boundary contained within the Exshaw Formation, Canada, determined an age of 361.3 ± 2.4 Ma. This age is in accord with U-Pb dates of interbedded tuff horizons and also U-Pb zircon date for the type Devonian-Mississippian Hasselbachtal section, Germany. The agreement of the biostratigraphic and U-Pb constraints of the Exshaw Formation with the Re-Os date illustrated the potential of the Re-Os chronometer to yield age determinations for sedimentary packages, especially in the absence of interbedd tuff horizons and biozones. A Re-Os date for the proposed type section of the Oxfordian-Kimmeridgian boundary, Staffin Bay, Isle of Skye, U.K., gave an age of 154.1 ± 2.2 Ma. This Re-Os age presents a 45 % (1.8 Ma) improvement in precision for the basal Kimmeridgian. It also demonstrated that the duration of the Kimmeridgian is nominally 3.3 Ma and thus is 1.6 Ma shorter than previously indicated. In addition to these examples, several studies have presented precise dates for Phanerozoic marine organic-rich units that are in excellent agreement with biostratigraphic determinations. A recent Re-Os study of the Woodford Shale (that was deposited throughout the Frasnian and Famennian) has provided important time markers as well as suggesting that the sedimentation rate of the Formation was relatively constant for ~20 Ma. To date only marine organic-rich sedimentary rocks have been utilized for Re-Os geochronology. However, lacustrine sedimentary rocks provide an invaluable archive of continental geological processes responding to tectonic, climatic and magmatic influences. Correlating these rocks to global geological phenomena requires accurate geochronological frameworks. The organic-rich lacustrine sedimentary units of the Eocene Green River Formation are enriched is Re and Os comparable to that of marine units. The Re-Os dates for the Green River Formation from the Uinta basin are 48.5 ± 0.6 Ma and 49.2 ± 1.0 Ma. These dates are in excellent agreement with Ar/Ar and U/Pb dates of interbedded tuffs in the GRF, therefore demonstrating that lacustrine units can be used for Re-Os geochronology in addition to marine organic-rich units.

Late Cretaceous porphyry copper mineralization in Sonora, Mexico: Implications for the evolution of the Southwest North America porphyry copper province

NASA Astrophysics Data System (ADS)

Barra, Fernando; Valencia, Victor A.

2014-10-01

Two porphyry Cu-Mo prospects in northern Sonora, Mexico (Fortuna del Cobre and Los Humos) located within the southwestern North American porphyry province have been dated in order to constrain the timing of crystallization and mineralization of these ore deposits. In Fortuna del Cobre, the pre-mineralization granodiorite porphyry yielded an U-Pb zircon age of 76.5 ± 2.3 Ma, whereas two samples from the ore-bearing quartz feldespathic porphyry were dated at 74.6 ± 1.3 and 75.0 ± 1.4 Ma. Four molybdenite samples from Los Humos porphyry Cu prospect yielded a weighted average Re-Os age of 73.5 ± 0.2 Ma, whereas two samples from the ore-bearing quartz monzonite porphyry gave U-Pb zircon ages of 74.4 ± 1.1 and 74.5 ± 1.3 Ma, showing a Late Cretaceous age for the emplacement of this ore deposit. The results indicate that Laramide porphyry Cu mineralization of Late Cretaceous age is not restricted to northern Arizona as previously thought and provide evidence for the definition of NS trending metallogenic belts that are parallel to the paleo-trench. Porphyry copper mineralization follows the inland migration trend of the magmatic arc as a result of the Farallon slab flattening during the Laramide orogeny.

U-Pb Geochronology of Grandite Skarn Garnet: Case Studies From Jurassic Skarns of California

NASA Astrophysics Data System (ADS)

Gevedon, M. L.; Seman, S.; Barnes, J.; Stockli, D. F.; Lackey, J. S.

2016-12-01

We present 3 case studies using a new method for U-Pb dating grossular-andradite (grandite) skarn garnet via LA-ICP-MS (Seman et al., in prep). Grandite is commonly rich in U, with high Fe3+ contents generally correlating with higher U concentrations. Micron-scale non-radiogenic Pb heterogeneities allow for regression of age data using Tera-Wasserberg concordia. Although others have dated accessory skarn minerals, garnet U-Pb ages are powerful because garnet grows early and is nearly ubiquitous in skarns, resists alteration, and provides a formation age independent of that of the causative pluton. The Darwin stock (Argus range, eastern CA) was likely a short-lived, single pulse of magmatism, genetically related to the Darwin skarn. A robust skarn garnet U-Pb age of 176.8 ± 1.3 Ma agrees well with the pluton U-Pb zircon age of 175 Ma (Chen and Moore, 1982). Furthermore, zircon separated from, and in textural equilibrium with, exoskarn garnetite yields a U-Pb age of 176.8 ± 1 Ma. Such agreement between plutonic and skarn zircon ages with a skarn garnet age in a geologically simple field area is the ideal scenario for establishing grandite U-Pb as a viable tool for directly dating skarns. The Black Rock skarn (BRS; eastern CA) is more complex: multiple plutons and ambiguous field relations complicate determination of a causative pluton. A skarn garnet U-Pb age of 172.0 ± 3 Ma confirms a middle Jurassic BRS formation age. Investigation of 4 local plutons yield zircon U-Pb ages of 222 ± 3 Ma, 213 ± 4 Ma, 207 ± 4 Ma and 176.2 ± 2 Ma. Comparison of the skarn garnet U-Pb and pluton ages suggest the BRS is genetically related to the youngest pluton, providing basis for further field and geochemical investigation. The Whitehorse skarn (WS; Mojave Desert, CA) lies in an important region for studying the changing tectono-magmatic regime of the Jurassic North American Cordillera; basin fill suggests a tectonically-controlled oscillating regional shoreline (Busby, 2012). Values of δ18O of WS garnet are strongly negative, —9.8‰ to 1.2‰, and can only be the result of access to meteoric water. A grandite U-Pb age of 162.3 ± 2 Ma requires the region of the WS not be submerged below the sea at this time. Future work coupling grandite U-Pb and δ18O may be vital in establishing the timing of Jurassic Cordilleran tectonic changes in the Mojave Desert.

U-Pb thermochronology of rutile from Alpine Corsica: constraints on the thermal evolution of the European margin during Jurassic continental breakup

NASA Astrophysics Data System (ADS)

Ewing, T. A.; Beltrando, M.; Müntener, O.

2017-12-01

U-Pb thermochronology of rutile can provide valuable temporal constraints on the exhumation history of the lower crust, given its moderate closure temperature and the occurrence of rutile in appropriate lithologies. We present an example from Alpine Corsica, in which we investigate the thermal evolution of the distal European margin during Jurassic continental rifting that culminated in the opening of the Alpine Tethys ocean. The Belli Piani unit of the Santa Lucia nappe (Corsica) experienced minimal Alpine overprint and bears a striking resemblance to the renowned Ivrea Zone lower crustal section (Italy). At its base, a 2-4 km thick gabbroic complex contains slivers of granulite facies metapelites that represent Permian lower crust. Zr-in-rutile temperatures and U-Pb ages were determined for rutile from three metapelitic slivers from throughout the Mafic Complex. High Zr-in-rutile temperatures of 850-950 °C corroborate textural evidence for rutile formation during Permian granulite facies metamorphism. Lower Zr-in-rutile temperatures of 750-800 °C in a few grains are partly associated with elongate strings of rutile within quartz ribbons, which record recrystallisation of some rutile during high-temperature shearing. Zr thermometry documents that both crystallisation and re-crystallisation of rutile occurred above the closure temperature of Pb in rutile, such that the U-Pb system can be expected to record cooling ages uncomplicated by re-crystallisation. Our new high-precision single-spot LA-ICPMS U-Pb dates are highly consistent between and within samples. The three samples gave ages from 160 ± 1 Ma to 161 ± 2 Ma, with no other age populations detected. The new data indicate that the Santa Lucia lower crust last cooled through 550-650 °C at 160 Ma, coeval with the first formation of oceanic crust in the Tethys. The new data are compared to previous depth profiling rutile U-Pb data for the Belli Piani unit1, and exploited to cast light on the tectonothermal evolution of the Santa Lucia lower crust in the Jurassic. The new data integrated with published data from the Ivrea zone allow comparison of the thermal evolution of the opposing European (Santa Lucia) and Adriatic (Ivrea) continental margins created by rifting associated with the opening of the Tethys. 1Seymour NM et al., 2016, Tectonics 35, 2439-2466

Age of the moon: An isotopic study of uranium-thorium-lead systematics of lunar samples

USGS Publications Warehouse

Tatsumoto, M.; Rosholt, J.N.

1970-01-01

Concentrations of U, Th, and Pb in Apollo 11 samples studied are low (U. 0.16 to 0.87; Th, 0.53 to 3.4; Pb, 0.29 to 1.7, in ppm) but the extremely radiogenic lead in samples allows radiometric dating. The fine dust and the breccia have a concordant age of 4.66 billion years on the basis of 207Pb/206Pb, 206Pb/238U, 207Pb/235U, and 208Pb/232Th ratios. This age is comparable with the age of meteorites and with the age generally accepted for the earth. Six crystalline and vesicular samples are distinctly younger than the dust and breccia. The 238U/235U ratio is the same as that in earth rocks, and 234U is in radioactive equilibrium with parent 238U.

Exploring the U-Pb systematics of titanite from the Archean Stillwater Complex

NASA Astrophysics Data System (ADS)

Friedman, R. M.; Wall, C. J.; Scoates, J. S.; Weis, D. A.; Meurer, W. P.

2011-12-01

The Stillwater Complex is a large mafic-ultramafic layered intrusion in the Beartooth Mountains of Montana (USA) and host to the world-class J-M Reef platinum group element deposit. The size and geologic/economic importance of this igneous complex make it an important target for high-precision U-Pb dating. As a part of a comprehensive U-Pb study of the Stillwater Complex, we present ID-TIMS U-Pb titanite data, including new single grain results produced using the EARTHTIME ET535 spike, for very low-volume, relatively felsic granophyric and pegmatitic rocks associated with Stillwater layered rocks. Four samples studied include a pegmatitic ksp-qtz core to a gabbroic pegmatoid in the Lower Banded Series (N1), an alaskite (quartz diorite) and an amphibole-rich reaction zone between the alaskite and anorthosite (AN1) in the Middle Banded Series, and an amphibole-bearing granophyre from the Upper Banded Series (GN3). CA-TIMS U-Pb dating of zircon from these samples yielded concordant results only for the pegmatitic rock (weighted 207Pb/206Pb: 2709.65 ± 0.80 Ma, n = 5), which agrees with new zircon ages from Stillwater layered rocks. Results for high-U (up to 1438 ppm) metamict zircon that occurs in the other three rocks were highly discordant and did not yield precise ages. Titanite U-Pb results for the pegmatite are about -1% to +1% discordant with two groupings of 207Pb/206Pb dates: one with a weighted average of 2708.1 ± 2.0 Ma (n = 2), which overlaps in age with zircon from the same sample and the crystallization age of the Stillwater Complex, and a second, younger grouping of 2701.1 ± 1.3 Ma (n = 5). Younger dates record an early Pb-loss event, possibly related to intrusion of cross-cutting quartz monzonites. The alaskite data also shows two groupings of 207Pb/206Pb dates, although more subtle: a weighted average of 2709.3 ± 1.8 Ma (n = 3) and a single result of 2706.5 ± 1.7 Ma. Titanite from the other two samples has undergone significant Pb-loss. Results for four analyses from the reaction zone sample are 1.74-54.3% discordant and lie in a quasi-linear array with intercepts at ca. 2700 Ma and 325 Ma. Data for the granophyre in the Upper Banded Series also have a wide range of discordance (1.5-3.4%, 21.4% and 61.3%). Titanites from these samples appear to have undergone a two stage Pb-loss history: an early, relatively minor event as suggested for the pegmatite, and a subsequent episode, which for some grains resulted in significant discordance. Concordant U-Pb titanite data from two of the Stillwater Complex granophyres confirms nearly synchronous crystallization with that of the layered rocks in the intrusion. For the pegmatite, the ages and overlap of zircon and titanite suggest a direct relationship between the granophyres and the layered rock and relatively rapid cooling of the complex through titanite closure temperature. Titanite from the alaskite yields useful age information, whereas coexisting zircon are highly discordant and not age diagnostic.

Air abrasion experiments in U-Pb dating of zircon

USGS Publications Warehouse

Goldich, S.S.; Fischer, L.B.

1986-01-01

Air abrasion of zircon grains can remove metamict material that has lost radiogenic Pb and zircon overgrowths that were added during younger events and thereby improve the precision of the age measurements and permit closer estimates of the original age. Age discordance that resulted from a single disturbance of the U-Pb isotopic decay systems, as had been demonstrated by T.E. Krogh, can be considerably reduced, and, under favorable conditions, the ages brought into concordancy. Two or more events complicate the U-Pb systematics, but a series of abrasion experiments can be helpful in deciphering the geologic history and in arriving at a useful interpretation of the probable times of origin and disturbances. In east-central Minnesota, U.S.A., Penokean tonalite gneiss is dated at 1869 ?? 5 Ma, and sheared granite gneiss is shown to have been a high-level granite intrusion at 1982 ?? 5 Ma in the McGrath Gneiss precursor. Tonalite gneiss and a mafic granodiorite in the Rainy Lake area, Ontario, Canada, are dated at 2736 ?? 16 and 2682 ?? 4 Ma, respectively. The tonalitic phase of the Morton Gneiss, southwestern Minnesota, is dated at 3662 ?? 42 Ma. ?? 1986.

U-Pb zircon geochronology and Zr-in-rutile thermometry of eclogites from the Dulan area, North Qaidam ultra-high pressure (UHP) terrane, western China

NASA Astrophysics Data System (ADS)

Hernández Uribe, D.; Stubbs, K.; Lehman, M. R.; Gilmore, V.; Kylander-Clark, A. R.; Mattinson, C. G.

2016-12-01

The Dulan area, in the North Qaidam terrane, exposes UHP eclogites and gneisses that experienced a 20 Myr UHP event at P-T conditions of 30 kbar and 700 °C. Two eclogites were analyzed using Zr-in-rutile thermometry and zircon U-Pb + trace element analysis to constrain the metamorphic evolution of the area. A kyanite-phengite eclogite presents a mineral assemblage of grt + omp + ph + ky + rt + zo + qz. Rutile analyses show a Zr concentration of 173-250 ppm with a mean of 207 ± 19 ppm. The calculated temperatures yielded 685-716 °C with an average of 700 ± 7°C. Zircon U-Pb analyses gave an upper intercept age of 880 ± 89 Ma. These analyses from cathodoluminiscence (CL)-dark core zircons show a negative Eu anomaly and a steep HREE slope suggesting a magmatic origin for the protolith. Analyses from CL-bright rims gave a weighted mean age of 427 ± 2 Ma. These zircons show an eclogite facies trace elements pattern suggesting that the age represent the HP-UHP event. Titanium concentration in zircons gave a weighted mean of 4.41 ± 0.25 ppm. This Ti concentration yielded a calculated temperature of 674 °C A phengite eclogite shows a mineral assemblage of grt + omp + ph + rt + zo + qz. Rutile in matrix analyses show a Zr concentration of 123-161 ppm with a mean of 139 ± 9 ppm. Calculated temperatures for these rutiles ranges from 659-680 °C with a mean temperature of 668 ± 5 °C. U-Pb analyses from CL-dark zircon cores gave a weighted mean age of 844 ± 7 Ma. These zircons show a negative Eu anomaly and a steep HREE slope suggesting a magmatic origin for the protolith. Analyses from CL-grey rims gave a weighted mean age of 433 ± 4 Ma. These zircons show an eclogite facies trace elements pattern, representing the timing of the HP-UHP event. Titanium concentration in zircons gave a weighted mean of 3.13 ± 0.34 ppm. This concentration yielded calculated temperature 647 °C. The obtained ages are in the same range as the ones obtained for the northern and southern Dulan area suggesting that all the area belongs to the same tectonic unit and experienced the same continental collision metamorphism.

Advantages of conducting in-situ U-Pb age dating of multiple U-bearing minerals from a single complex: Case in point - the Oka Carbonatite Complex

NASA Astrophysics Data System (ADS)

Chen, W.; Simonetti, A.

2012-12-01

A detailed radiometric investigation is currently underway focusing on U-bearing accessory minerals apatite, perovskite, and niocalite from the Oka Carbonatite Complex (Canada). One of the main objectives is to obtain a comparative chronology of melt crystallization for the complex. Unlike other commonly adopted U-bearing minerals (e.g., zircon, monazite) for in-situ dating investigations, apatite, perovskite, and niocalite contain relatively high contents of common Pb. Hence, careful assessment of the proportion and composition of the common Pb, and usage of appropriate matrix-matched external standards are imperative. The Madagascar apatite was utilized as the external standard for apatite dating, and the Emerald Lake and Durango apatites were adopted as secondary standards; the latter yield ages of 92.6 ±1.8 and 32.2 ±1.1 Ma, respectively, and these are identical to their accepted ages. Pb/U ages for apatite from Oka were obtained for different rock types, including 8 carbonatites, 4 okaites, 3 ijolites and 3 alnoites, and these define a range of ages between ~105 and ~135 Ma; this result suggests a protracted crystallization history. In total, 266 individual analyses define two peaks at ~115 and ~125Ma. For perovskite dating, the Ice River perovskite standard was utilized as the external standard. The perovskites from one okaite sample yield an age of 112.2 ±1.9 Ma, and is much younger than the previously reported U-Pb perovskite age of 131 ±7 Ma. Hence, the combined U-Pb perovskite ages also suggest a rather prolonged time of melt crystallization. Niocalite is a rare, accessory silicate mineral that occurs within the carbonatites at Oka. The international zircon standard BR266 was selected for use as the external standard and rastering was employed to minimize the Pb-U fractionation. Two niocalite samples give young ages at 110.6 ±1.2 and 115.0 ±1.9 Ma, and are identical to their respective apatite ages (given associated uncertainties) from the same sample. The niocalite for a carbonatite sample Oka153 defines a bimodal age distribution, with weighted average 206Pb/238U ages of 110.1 ±5.0 and 133.2 ±6.1 Ma. Apatite from the same sample also records a similar bimodal age distribution of 111.4 ±2.8 and 126.9 ±1.8 Ma. The combined in situ U-Pb dating results for apatite, pervoskite, niocalite from Oka clearly support a protracted history of magmatic activity (~30 Myr) for this carbonatite complex. Of importance, the U-Pb results from this study clearly indicate the significance of conducting a thorough geochronological investigation rather than defining the age of any one alkaline complex solely on the basis of a single radiometric age determination.

Reply to Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on the 0.1 Ma Toya Tephra, Japan"

NASA Astrophysics Data System (ADS)

Ito, Hisatoshi

2015-04-01

Guillong et al. (2015) mentioned that corrections for abundance sensitivity for 232Th and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon for LA-ICP-MS analyses. There is no denying that more rigorous treatments are necessary to obtain more reliable ages than those in Ito (2014). However, as shown in Fig. 2 in Guillong et al. (2015), the uncorrected (230Th)/(238U) for reference zircons except for Mud Tank are only 5-20% higher than unity. Since U abundance of Toya Tephra zircons that have U-Pb ages < 1 Ma is in-between that of FCT and Plesovice, the overestimation of 230Th by both abundance sensitivity and molecular interferences is expected to be 5-20% for the Toya Tephra. Moreover Ito (2014) obtained U-Th ages of the Toya Tephra by comparison with Fish Canyon Tuff (FCT) data. Because both the FCT and the Toya Tephra have similar trends of overestimation of 230Th, the effect of overestimation of 230Th to cause overestimation of U-Th age should be cancelled out or negligible. Therefore the pivotal conclusion in Ito (2014) that simultaneous U-Pb and U-Th dating using LA-ICP-MS is possible and useful for Quaternary zircons holds true.

New zircon (U-Th)/He and U/Pb eruption age for the Rockland tephra, western USA

USGS Publications Warehouse

Coble, Matthew A.; Burgess, Seth; Klemetti, Erik W.

2017-01-01

Eruption ages of a number of prominent Quaternary volcanic deposits remain inaccurately and/or imprecisely constrained, despite their importance as regional stratigraphic markers in paleo-environment reconstruction and as evidence of climate-altering eruptions. Accurately dating volcanic deposits presents challenging analytical considerations, including poor radiogenic yield, scarcity of datable minerals, and contamination of crystal populations by magma, eruption, and transport processes. One prominent example is the Rockland tephra, which erupted from the Lassen Volcanic Center in the southern Cascade arc. Despite a range in published eruption ages from 0.40 to 0.63 Ma, the Rockland tephra is extensively used as a marker bed across the western United States. To more accurately and precisely constrain the age of the Rockland tephra-producing eruption, we report U/Pb crystallization dates from the outermost ∼2 μm of zircon crystal faces (surfaces) using secondary ion mass spectrometry (SIMS). Our new weighted mean 238U/206Pb age for Rockland tephra zircon surfaces is 0.598 ± 0.013 Ma (2σ) and MSWD = 1.11 (mean square weighted deviation). As an independent test of the accuracy of this age, we obtained new (U-Th)/He dates from individual zircon grains from the Rockland tephra, which yielded a weighted mean age of 0.599 ± 0.012 Ma (2σ, MSWD = 5.13). We also obtained a (U-Th)/He age of 0.628 ± 0.014 Ma (MSWD = 1.19) for the Lava Creek Tuff member B, which was analyzed as a secondary standard to test the accuracy of the (U-Th)/He technique for Quaternary tephras, and to evaluate assumptions made in the model-age calculation. Concordance of new U/Pb and (U-Th)/He zircon ages reinforces the accuracy of our preferred Rockland tephra eruption age, and confirms that zircon surface dates sample zircon growth up to the time of eruption. We demonstrate the broad applicability of coupled U/Pb zircon-surface and single-grain zircon (U-Th)/He geochronology to accurate dating of Quaternary tephra, and highlight the challenges and opportunities of this technique.

New zircon (U-Th)/He and U/Pb eruption age for the Rockland tephra, western USA

NASA Astrophysics Data System (ADS)

Coble, Matthew A.; Burgess, Seth D.; Klemetti, Erik W.

2017-09-01

Eruption ages of a number of prominent Quaternary volcanic deposits remain inaccurately and/or imprecisely constrained, despite their importance as regional stratigraphic markers in paleo-environment reconstruction and as evidence of climate-altering eruptions. Accurately dating volcanic deposits presents challenging analytical considerations, including poor radiogenic yield, scarcity of datable minerals, and contamination of crystal populations by magma, eruption, and transport processes. One prominent example is the Rockland tephra, which erupted from the Lassen Volcanic Center in the southern Cascade arc. Despite a range in published eruption ages from 0.40 to 0.63 Ma, the Rockland tephra is extensively used as a marker bed across the western United States. To more accurately and precisely constrain the age of the Rockland tephra-producing eruption, we report U/Pb crystallization dates from the outermost ∼2 μm of zircon crystal faces (surfaces) using secondary ion mass spectrometry (SIMS). Our new weighted mean 238U/206Pb age for Rockland tephra zircon surfaces is 0.598 ± 0.013 Ma (2σ) and MSWD = 1.11 (mean square weighted deviation). As an independent test of the accuracy of this age, we obtained new (U-Th)/He dates from individual zircon grains from the Rockland tephra, which yielded a weighted mean age of 0.599 ± 0.012 Ma (2σ, MSWD = 5.13). We also obtained a (U-Th)/He age of 0.628 ± 0.014 Ma (MSWD = 1.19) for the Lava Creek Tuff member B, which was analyzed as a secondary standard to test the accuracy of the (U-Th)/He technique for Quaternary tephras, and to evaluate assumptions made in the model-age calculation. Concordance of new U/Pb and (U-Th)/He zircon ages reinforces the accuracy of our preferred Rockland tephra eruption age, and confirms that zircon surface dates sample zircon growth up to the time of eruption. We demonstrate the broad applicability of coupled U/Pb zircon-surface and single-grain zircon (U-Th)/He geochronology to accurate dating of Quaternary tephra, and highlight the challenges and opportunities of this technique.

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

USGS Publications Warehouse

Matthews, Naomi E.; Vazquez, Jorge A.; Calvert, Andrew T.

2015-01-01

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar/39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103−104 year interval before eruption.

Provenance of the lower Miocene of the Gulf of Mexico from detrital zircon double dating

NASA Astrophysics Data System (ADS)

xu, J.

2013-12-01

The lower Miocene interval of the Gulf of Mexico (GOM) has recently gained increasing attention from oil and gas industry due to its hydrocarbon potential below the salt canopy. However, it has been less well studied than both the underlying Oligocene and overlying middle Miocene strata. The lower Miocene worldwide is a transitional period of tectonic, climatic, and oceanographic change. In particular, it is a period of major tectonic reorganization in the western interior of North America (Rocky Mountains), involving a shift from the Oligocene thermal phase, with abundant volcanic activity recorded in the thick Frio/Vicksburg succession of the GOM, to the Miocene Basin-Range extensional phase. Climatic conditions also changed from a relatively arid Oligocene to wetter Miocene, resulting in increased sediment yields from exhumed tectonic structures. Previous provenance studies used proportions of quartz, feldspar and lithic fragments and consideration of likely river courses through known paleogeomorphological elements. Only limited detrital zircon (DZ) U-Pb studies on Paleocene strata have been undertaken and there has been no previous U-Pb and (U-Th)/He double dating in the GOM. In this study we apply the latest analytical approaches, such as DZ U-Pb dating to gain robust source terranes ages and more fully elucidate the complex sediment provenance and dispersal history of GOM. We also employ DZ (U-Th)/He (ZHe) dating, combined with DZ U-Pb, to not only define sedimentary provenance but also the exhumation histories of detrital source regions. Samples of lower Miocene outcrop exposures in Texas and Louisiana have been collected to discriminate the varied tectonic and drainage system changes across the basin in lateral. In addition, samples from the Eocene, Oligocene and middle Miocene have been obtained to reveal vertical shift of source terranes contributions. Our initial age data show detrital zircons of lower Miocene sediments come from a wide range of source terranes including a large populations from the western interior of North America (Rocky Mountains), Grenville, Mid-Continent, and Yavapai-Mazatzal provinces, with smaller populations from the Appalachian-Ouachita, Wyoming or Superior regions. Based on U-Pb dating results, we will carry out (U-Th)/He dating on selected zircons to reveal the detailed exhumation histories of the sediment source regions. Using the dual criteria of DZ crystallization age (U-Pb) and cooling age (U-Th/He) to constrain provenance will enable us to generate rigorous reconstructions of the lower Miocene depositional systems from source terrane to deep-water sink for this key transitional period in geologic history.

Late Proterozoic charnockites in Orissa, India: A U-Pb and Rb-Sr isotopic study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Aftalion, M.; Bowes, D.R.; Dash, B.

1988-11-01

Charnockite formation in the Angul district of Orissa took place between 1088 + 26/ -17 Ma, the U-Pb zircon upper intercept crystallization age of a leptynite neosome, and 957 +8/ -4-956 {plus minus} 4 Ma, the U-Pb zircon-monazite upper intercept and U-Pb monazite crystallization ages of a granite. Confirmation of the Proterozoic age of the charnockites is given by (1) a U-Pb zircon upper intercept 1159 + 59/ -30 Ma age and a Rb-Sr whole-rock 1080 {plus minus} 65 Ma age for an augen gneiss which pre-dates the leptynite, and (2) U-Pb monazite ages of 973 {plus minus} 5,964 {plusmore » minus} 4, and 953 {plus minus} 4 Ma for a gray quartzofeldspathic gneiss, the augen gneiss, and the leptynite, respectively: these late Proterozoic dates are interpreted as representing ages recorded during charnockitization. The ca. 950-980 Ma charnockite- and granite-forming events are related to the evolution of mantle-derived, CO{sub 2}-bearing basic magma emplaced into the deeper levels of an extensional tectonic-transcurrent fault regime. The ca. 1100-1150 Ma tectonothermal and igneous events represent compressional tectonism in reactivated crystalline basement in the late mid-Proterozoic Eastern Ghats orogenic belt.« less

Variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite NWA 5298 and improved U-Pb dating of Solar System events

NASA Astrophysics Data System (ADS)

Darling, James R.; Moser, Desmond E.; Barker, Ivan R.; Tait, Kim T.; Chamberlain, Kevin R.; Schmitt, Axel K.; Hyde, Brendt C.

2016-06-01

The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U-Pb isotopic systematics, but there is little understanding of shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate shock-effects and their linkage with U-Pb isotopic disturbance that has previously been documented by in-situ U-Pb isotopic analyses. The shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U-Pb isotope compositions. Primary igneous features in Group 1 baddeleyites (n = 5) are retained in high shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U-Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in baddeleyite results from a single cycle of shock-loading, heating and cooling during transit to space, and that this leads to variable disturbance of the U-Pb isotope system. Furthermore, by linking in-situ U-Pb isotopic measurements with detailed micro- to nano-structural analyses, it is possible to resolve the timing of both endogenic crustal processes and impact events in highly-shocked planetary materials using baddeleyite. This opens up new opportunities to refine the timing of major events across the Solar System.

Fallout isotope chronology of the near-surface sediment record of Lake Bolătău.

PubMed

Bihari, Árpád; Karlik, Máté; Mîndrescu, Marcel; Szalai, Zoltán; Grădinaru, Ionela; Kern, Zoltán

2018-01-01

Fallout isotope ( 210 Pb ex, 137 Cs and 241 Am) based dating has been carried out on the near-surface sediment core collected from Lake Bolătău-Feredeu (Bukovina, Romania). The motivation was to improve the chronology of this recent section in connection with significant fluctuations observed in sediment accumulation rates, particle size distribution and primordial radioisotope (i.e. 40 K and 232 Th) composition. Previously only an extrapolation of a broad-range OxCal age-depth model, which was based on 8 AMS radiocarbon dates from the deeper part of a parallel sediment sequence and tentatively validated for the upper part using the double peaks of the 137 Cs activity concentration distribution, was available for the studied section (1-24 cm). Parallel to the previous 137 Cs measurement, 210 Pb and 226 Ra (for a more detailed, 210 Pb ex -based chronology), 241 Am (for an additional time-marker), as well as 40 K and 232 Th concentrations have also been determined by gamma-spectrometry. In case of the 210 Pb ex -based chronology, due to a large deviation from a pure exponential distribution, the Constant Flux (CF) model has been used for the calculation of sediment ages and accumulation rates. Although the broad-range OxCal and the CF model were broadly similar down to 22 cm, the 210 Pb ex -based ages are clearly superior in terms of uncertainty in the uppermost 12 cm, while the broad-range model has smaller uncertainty below 20 cm (>150 years). The CF model gave an average mass accumulation rate of (0.08 ± 0.03) g cm -2 yr -1 for sections 0-11 cm, and (0.03 ± 0.01) g cm -2 yr -1 for sections 12-22 cm, respectively. Significant changes have been observed in the depth distribution of both the particle size distribution and the elemental/isotopic composition of the sediment record, most likely related to the variation observable in the intensity and volume of precipitation in the catchment. The obtained high-resolution records of Lake Bolătău, including multiple radioisotopes, can serve as a regional benchmark for similar studies. Copyright © 2017 Elsevier Ltd. All rights reserved.

Laser Ablation in situ (U-Th-Sm)/He and U-Pb Double-Dating of Apatite and Zircon: Techniques and Applications

NASA Astrophysics Data System (ADS)

McInnes, B.; Danišík, M.; Evans, N.; McDonald, B.; Becker, T.; Vermeesch, P.

2015-12-01

We present a new laser-based technique for rapid, quantitative and automated in situ microanalysis of U, Th, Sm, Pb and He for applications in geochronology, thermochronometry and geochemistry (Evans et al., 2015). This novel capability permits a detailed interrogation of the time-temperature history of rocks containing apatite, zircon and other accessory phases by providing both (U-Th-Sm)/He and U-Pb ages (+trace element analysis) on single crystals. In situ laser microanalysis offers several advantages over conventional bulk crystal methods in terms of safety, cost, productivity and spatial resolution. We developed and integrated a suite of analytical instruments including a 193 nm ArF excimer laser system (RESOlution M-50A-LR), a quadrupole ICP-MS (Agilent 7700s), an Alphachron helium mass spectrometry system and swappable flow-through and ultra-high vacuum analytical chambers. The analytical protocols include the following steps: mounting/polishing in PFA Teflon using methods similar to those adopted for fission track etching; laser He extraction and analysis using a 2 s ablation at 5 Hz and 2-3 J/cm2fluence; He pit volume measurement using atomic force microscopy, and U-Th-Sm-Pb (plus optional trace element) analysis using traditional laser ablation methods. The major analytical challenges for apatite include the low U, Th and He contents relative to zircon and the elevated common Pb content. On the other hand, apatite typically has less extreme and less complex zoning of parent isotopes (primarily U and Th). A freeware application has been developed for determining (U-Th-Sm)/He ages from the raw analytical data and Iolite software was used for U-Pb age and trace element determination. In situ double-dating has successfully replicated conventional U-Pb and (U-Th)/He age variations in xenocrystic zircon from the diamondiferous Ellendale lamproite pipe, Western Australia and increased zircon analytical throughput by a factor of 50 over conventional methods.Reference: Evans NJ, McInnes BIA, McDonald B, Becker T, Vermeesch P, Danisik M, Shelley M, Marillo-Sialer E and Patterson D. An in situ technique for (U-Th-Sm)/He and U-Pb double dating. J Analytical Atomic Spectrometry, 30, 1636 - 1645.

Apatite U-Pb thermochronolgy applied to complex geological settings - insights from geo/thermochronology and geochemistry

NASA Astrophysics Data System (ADS)

Paul, Andre; Spikings, Richard; Ulyanov, Alexey; Chew, David

2016-04-01

Application of high temperature (>350oC) thermochronology is limited to the U-Pb system of accessory minerals, such as apatite, under the assumption that radiogenic lead is lost to thermally activated volume diffusion into an infinite reservoir. Cochrane et al. (2015) have demonstrated a working example from the northern Andes of South America. Predictions from volume diffusion theory were compared with measured single grain U-Pb date correlated to shortest diffusion radius and in-situ profiles measured by LA-ICP-MS. Results from both techniques were found to be in agreement with predictions from thermally activated, volume diffusion. However, outliers from the ID-TIMS data suggested some complexity, as grains were found to be too young relative to their diffusion radius. Interaction of multiple processes can be responsible for the alteration of apatite U-Pb dates such as: (1) metamorphic (over)growth, (2) fluid aided alteration/recrystallization and (3) metamictization and fracturing of the grain. Further, predictions from volume diffusion rely on the input parameters: (a) diffusivity, (b) activation energy and (c) shortest diffusion radius. Diffusivity and activation energy are potentially influenced by the chemical composition and subsequent changes in crystal structure. Currently there is one value for diffusion parameter and activation energy established for (Durango) apatite (Cherniak et al., 1991). Correlation between diffusivity/activation energy and composition has not been established. We investigate if correlations exist between diffusivity/activation energy and composition by obtaining single grain apatite U-Pb date and chemical compostion and correlating these to their diffusion radius. We test the consistency of apatite closure temperature, by comparing the apatite U-Pb dates with lower temperature thermochronometers such as white mica and K-feldspar Ar/Ar and by petrographic observations. We test if chemical information can be a proxy to identify metamorphic (over)growth and fluid aided alteration/recrystallization. We seek to evaluate if apatite U-Pb thermochronology can be applied to a broad range of rock types and geological environments or if limitations must be drawn.

In situ SIMS U-Pb dating of hydrothermal rutile: reliable age for the Zhesang Carlin-type gold deposit in the golden triangle region, SW China

NASA Astrophysics Data System (ADS)

Pi, Qiaohui; Hu, Ruizhong; Xiong, Bin; Li, Qiuli; Zhong, Richen

2017-12-01

The contiguous region between Guangxi, Guizhou, and Yunnan, commonly referred to as the Golden Triangle region in SW China, hosts many Carlin-type gold deposits. Previously, the ages of the gold mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating. This paper reports the first SIMS U-Pb age of hydrothermal rutile crystals for the Zhesang Carlin-type gold deposit in the region. The hydrothermal U-bearing rutile associated with gold-bearing sulfides in the deposit yields an U-Pb age of 213.6 ± 5.4 Ma, which is within the range of the previously reported arsenopyrite Re-Os isochron ages (204 ± 19 to 235 ± 33 Ma) for three other Carlin-type gold deposits in the region. Our new and more precise rutile U-Pb age confirms that the gold mineralization was contemporaneous with the Triassic W-Sn mineralization and associated granitic magmatism in the surrounding regions. Based on the temporal correlation, we postulate that coeval granitic plutons may be present at greater depths in the Golden Triangle region and that the formation of the Carlin-type gold deposits is probably linked to the coeval granitic magmatism in the region. This study clearly demonstrates that in situ rutile U-Pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile.

The Chachil Limestone (Pliensbachian-earliest Toarcian) Neuquén Basin, Argentina: U-Pb age calibration and its significance on the Early Jurassic evolution of southwestern Gondwana

NASA Astrophysics Data System (ADS)

Leanza, H. A.; Mazzini, A.; Corfu, F.; Llambías, E. J.; Svensen, H.; Planke, S.; Galland, O.

2013-03-01

New radiometric U-Pb ages obtained on zircon crystals from Early Jurassic ash layers found within beds of the Chachil Limestone at its type locality in the Chachil depocentre (southern Neuquén Basin) confirm a Pliensbachian age (186.0 ± 0.4 Ma). Additionally, two ash layers found in limestone beds in Chacay Melehue at the Cordillera del Viento depocentre (central Neuquén Basin) gave Early Pliensbachian (185.7 ± 0.4 Ma) and earliest Toarcian (182.3 ± 0.4 Ma) U-Pb zircon ages. Based on these new datings and regional geological observations, we propose that the limestones cropping out at Chacay Melehue are correlatable with the Chachil Limestone. Recent data by other authors from limestones at Serrucho creek in the upper Puesto Araya Formation (Valenciana depocentre, southern Mendoza) reveal ages of 182.16 ± 0.6 Ma. Based on these new evidences, we consider the Chachil Limestone an important Early Jurassic stratigraphic marker, representing an almost instantaneous widespread flooding episode in western Gondwana. The unit marks the initiation in the Neuquén Basin of the Cuyo Group, followed by widespread black shale deposition. Accordingly, these limestones can be regarded as the natural seal of the Late Triassic -earliest Jurassic Precuyano Cycle, which represents the infill of halfgrabens and/or grabens related to a strong extensional regime. Paleontological evidence supports that during Pliensbachian-earliest Toarcian times these limestones were deposited in western Gondwana in marine warm water environments.

The 238U/235U isotope ratio of the Earth and the solar system: Constrains from a gravimetrically calibrated U double spike and implications for absolute Pb-Pb ages

NASA Astrophysics Data System (ADS)

Weyer, Stefan; Noordmann, Janine; Brennecka, Greg; Richter, Stephan

2010-05-01

The ratio of 238U and 235U, the two primordial U isotopes, has been assumed to be constant on Earth and in the solar system. The commonly accepted value for the 238U/235U ratio, which has been used in Pb-Pb dating for the last ~ 30 years, was 137.88. Within the last few years, it has been shown that 1) there are considerable U isotope variations (~1.3‰) within terrestrial material produced by isotope fractionation during chemical reactions [1-3] and 2) there are even larger isotope variations (at least 3.5‰) in calcium-aluminum-rich inclusions (CAIs) in meoteorites that define the currently accepted age of the solar system [4]. These findings are dramatic for geochronology, as a known 238U/235U is a requirement for Pb-Pb dating, the most precise dating technique for absolute ages. As 238U/235U variations can greatly affect the reported absolute Pb-Pb age, understanding and accurately measuring variation of the 238U/235U ratio in various materials is critical, With these new findings, the questions also arises of "How well do we know the average U isotope composition of the Earth and the solar system?" and "How accurate can absolute Pb-Pb ages be?" Our results using a gravimetrically calibrated 233U/236U double spike IRMM 3636 [5] indicate that the U standard NBL 950a, which was commonly used to define the excepted "natural" 238U/235U isotope ratio, has a slightly lower 238U/235U of 137.836 ± 0.024. This value is indistinguishable from the U isotope compositions for NBL 960 and NBL112A, which have been determined by several laboratories, also using the newly calibrated U double spike IRMM 3636 [6]. These findings provide new implications about the average U isotope composition of the Earth and the solar system. Basalts display a very tight range of U isotope variations (~0.25-0.32‰ relative to SRM 950a). Their U isotope composition is also very similar to that of chondrites [4], which however appear to show a slightly larger spread. Accepting terrestrial basalts to be the best representation of the U isotope composition of the Earth and the solar system, and the new 238U/235U for SRM 950a of 137.836, this would result in an average 238U/235U for the Earth and the solar system of ~ 137.80. The effect of a revised 238U/235U ratio on Pb-Pb ages is age dependent. It results in an age overestimation of ~0.8 Ma for the age of the solar system and up to 1.5 Ma for very young material (with bulk Earth U isotope composition). [1] Stirling, C.H., Andersen, M.B., Potter, E.-K., et al. (2007) EPSL 264, 208-225; [2] Weyer, S., Anbar, A. D., Gerdes, A., et al. (2008) GCA 72, 345-359. [3] Bopp, C.H., Lundstrom, C.C., Johnsons, T.M., Glessner, J.G. (2009) Geology 37, 611-614. [4] Brennecka, G.A., Weyer, S., Wadhwa, et al. (2010) Science, 327, p. 449. [5] Richter, S., Alonzo-Munoz, A., Eykens, R., et al. (2008) IJMS 269, 145-148. [6] Richter, S., et al. EGU meeting 2010.

Precise U/Pb zircons dates of bentonites in Upper Ordovician and Lower Silurian reference sections in North America and Britain.

NASA Astrophysics Data System (ADS)

Suarez, S. E.; Brookfield, M. E.; Catlos, E. J.; Stockli, D. F.; Batchelor, R. A.

2016-12-01

The end of the Ordovician marks one of the greatest of the Earth's mass extinctions. One hypothesis explains this mass extinction as the result of a short-lived, major glaciation preceded by episodes of increased volcanism brought on by the Taconic orogeny. K-bentonites, weathered volcanic ash, provide evidence for increased volcanism. However, there is a lack of modern precise U-Pb dating of these ashes and some confusion in the biostratigraphy. The aim of this study is to obtain more precise U-Pb zircon ages from biostratigraphically constrained bentonites which will lead to better correlation of the Upper Ordovician and Lower Silurian relative time scales, as well as time the pulses of eruption. Zircon grains were extracted from the samples by heavy mineral separation and U-Pb dated using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer at the University of Texas-Austin. We report here 3 precise U-Pb zircon ages from the Trenton Group, Ontario, Canada, and Dob's Linn, Scotland. The youngest age from the top of the Kirkfield Formation in Ontario is 448.0 +/- 18 Ma, which fits with existing late Ordovician stratigraphic ages. At Dob's Linn, Scotland, the site of the Ordovician/Silurian Global Boundary Stratigraphic Section and Point (GSSP), the youngest age for DL7, a bentonite 5 meters below the GSSP is 402.0 +/- 12.0 Ma, and for DL24L, a bentonite 8 meters above the GSSP is 358.2 +/- 7.9 Ma. These are Devonian ages in current timescales - the current age for the GSSP is 443.8 +/- 1.8 Ma, based on an U/Pb dates from a bentonite 1.6 meters above the GSSP at Dob's Linn. We are confident that our techniques rule out contamination and the most likely explanation is that the small zircons we analyzed either suffered Pb loss, or grew overgrowths during low grade hydrothermal metamorphism of the sediments during the intrusion of the Southern Upland Devonian granites during the Caledonian orogeny. These Devonian ages suggest that the 443.8 +/- 1.8 Ma age may also be suspect. The Dob's Linn site is therefore unsuitable for calibrating the biostratigraphic horizons. Work in progress will provide more U-Pb dating of bentonites from around the Ordovician-Silurian boundary in Canada, United States, Britain and Scandinavia with the aim of calibrating the local series and stages in order to help in International correlations.

Titanite chronology, thermometry, and speedometry of ultrahigh-temperature (UHT) calc-silicates from south Madagascar: U-Pb dates, Zr temperatures, and lengthscales of trace-element diffusion

NASA Astrophysics Data System (ADS)

Holder, R. M.; Hacker, B. R.

2017-12-01

Calc-silicate rocks are often overlooked as sources of pressure-temperature-time data in granulite-UHT metamorphic terranes due to the strong dependence of calc-silicate mineral assemblages on complex fluid compositions and a lack of thermodynamic data on common high-temperature calc-silicate minerals such as scapolite. In the Ediacaran-Cambrian UHT rocks of southern Madagascar, clinopyroxene-scapolite-feldspar-quartz-zircon-titanite calc-silicate rocks are wide-spread. U-Pb dates of 540-520 Ma from unaltered portions of titanite correspond to cooling of the rocks through upper-amphibolite facies and indicate UHT metamorphism occurred before 540 Ma. Zr concentrations in these domains preserve growth temperatures of 900-950 °C, consistent with peak temperatures calculated by pseudosection modeling of nearby osumilite-bearing gneisses. Younger U-Pb dates (510-490 Ma) correspond to fluid-mediated Pb loss from titanite grains, which occurred below their diffusive Pb-closure temperature, along fractures. The extent of fluid alteration is seen clearly in back-scattered electron images and Zr-, Al-, Fe-, Ce-, and Nb-concentration maps. Laser-ablation depth profiling of idioblastic titanite grains shows preserved Pb diffusion profiles at grain rims, but there is no evidence for Zr diffusion, indicating that it was effectively immobile even at UHT.

Complex Histories of Two Lunar Zircons as Evidenced by their Internal Structures and U-Pb Ages

NASA Technical Reports Server (NTRS)

Pidgeon, R. T.; Nemchin, A. A.; Meyer, Charles

2006-01-01

The U-Pb dating of lunar zircon by ion-microprobe provides a robust technique for investigating the timing of lunar events [1,2]. However, we have now identified two cases where the U-Pb systems in a single zircon show more than one age. These complex zircons provide new opportunities for extending our knowledge on the timing of events in the early history of the Moon.

The 230Th correction is the 1st priority for accurate dates of young zircons: U/Th partitioning experiments and measurements

NASA Astrophysics Data System (ADS)

Krawczynski, M.; McLean, N.

2017-12-01

One of the most accurate and useful ways of determining the age of rocks that formed more than about 500,000 years ago is uranium-lead (U-Pb) geochronology. Earth scientists use U-Pb geochronology to put together the geologic history of entire regions and of specific events, like the mass extinction of all non-avian dinosaurs about 66 million years ago or the catastrophic eruptions of supervolcanoes like the one currently centered at Yellowstone. The mineral zircon is often utilized because it is abundant, durable, and readily incorporates uranium into its crystal structure. But it excludes thorium, whose isotope 230Th is part of the naturally occurring isotopic decay chain from 238U to 206Pb. Calculating a date from the relative abundances of 206Pb and 238U therefore requires a correction for the missing 230Th. Existing experimental and observational constraints on the way U and Th behave when zircon crystallizes from a melt are not known precisely enough, and thus currently the uncertainty in dates introduced by they `Th correction' is one of the largest sources of systematic error in determining dates. Here we present preliminary results on our study of actinide partitioning between zircon and melt. Experiments have been conducted to grow zircon from melts doped with U and Th that mimic natural magmas at a range of temperatures, and compositions. Synthetic zircons are separated from their coexisting glass and using high precision and high-spatial-resolution techniques, the abundance and distribution of U and Th in each phase is determined. These preliminary experiments are the beginning of a study that will result in precise determination of the zircon/melt uranium and thorium partition coefficients under a wide variety of naturally occurring conditions. This data will be fit to a multidimensional surface using maximum likelihood regression techniques, so that the ratio of partition coefficients can be calculated for any set of known parameters. The results of this study will reduce the largest source of uncertainty in dating young zircons and improve the accuracy of U-Pb dates, improving our ability to tell time during geologic processes. The attainment of more accurate timing of the geologic timescale is important to geologists of all disciplines, from paleontology to planetary cosmochemistry to geobiology.

Quantifying time in sedimentary successions by radio-isotopic dating of ash beds

NASA Astrophysics Data System (ADS)

Schaltegger, Urs

2014-05-01

Sedimentary rock sequences are an accurate record of geological, chemical and biological processes throughout the history of our planet. If we want to know more about the duration or the rates of some of these processes, we can apply methods of absolute age determination, i.e. of radio-isotopic dating. Data of highest precision and accuracy, and therefore of highest degree of confidence, are obtained by chemical abrasion, isotope-dilution, thermal ionization mass spectrometry (CA-ID-TIMS) 238U-206Pb dating techniques, applied to magmatic zircon from ash beds that are interbedded with the sediments. This techniques allows high-precision estimates of age at the 0.1% uncertainty for single analyses, and down to 0.03% uncertainty for groups of statistically equivalent 206Pb/238U dates. Such high precision is needed, since we would like the precision to be approximately equivalent or better than the (interpolated) duration of ammonoid zones in the Mesozoic (e.g., Ovtcharova et al. 2006), or to match short feedback rates of biological, climatic, or geochemical cycles after giant volcanic eruptions in large igneous provinces (LIP's), e.g., at the Permian/Triassic or the Triassic/Jurassic boundaries. We also wish to establish as precisely as possible temporal coincidence between the sedimentary record and short-lived volcanic events within the LIP's. Precision and accuracy of the U-Pb data has to be traceable and quantifiable in absolute terms, achieved by direct reference to the international kilogram, via an absolute calibration of the standard and isotopic tracer solutions. Only with a perfect control on precision and accuracy of radio-isotopic data, we can confidently determine whether two ages of geological events are really different, and avoid mistaking interlaboratory or interchronometer biases for age difference. The development of unprecedented precision of CA-ID-TIMS 238U-206Pb dates led to the recognition of protracted growth of zircon in a magmatic liquid (see, e.g., Schoene et al. 2012), which then becomes transferred into volcanic ashes as excess dispersion of 238U-206Pb dates (see, e.g., Guex et al. 2012). Zircon is crystallizing in the magmatic liquid shortly before the volcanic eruption; we therefore aim at finding the youngest zircon date or youngest statistically equivalent cluster of 238U-206Pb dates as an approximation of ash deposition (Wotzlaw et al. 2013). Time gaps between last zircon crystallization and eruption ("Δt") may be as large as 100-200 ka, at the limits of analytical precision. Understanding the magmatic crystallization history of zircon is the fundamental background for interpreting ash bed dates in a sedimentary succession. Ash beds of different stratigraphic position and age my be generated within different magmatic systems, showing different crystallization histories. A sufficient number of samples (N) is therefore of paramount importance, not to lose the stratigraphic age control in a given section, and to be able to discard samples with large Δt - but, how large has to be "N"? In order to use the youngest zircon or zircons as an approximation of the age of eruption and ash deposition, we need to be sure that we have quantitatively solved the problem of post-crystallization lead loss - but, how can we be sure?! Ash bed zircons are prone to partial loss of radiogenic lead, because the ashes have been flushed by volcanic gases, as well as brines during sediment compaction. We therefore need to analyze a sufficient number of zircons (n) to be sure not to miss the youngest - but, how large has to be "n"? Analysis of trace elements or oxygen, hafnium isotopic compositions in dated zircon may sometimes help to distinguish zircon that is in equilibrium with the last magmatic liquid, from those that are recycled from earlier crystallization episodes, or to recognize zircon with partial lead loss (Schoene et al. 2010). Respecting these constraints, we may arrive at accurate correlation of periods of global environmental and biotic disturbance (from ash bed analysis in biostratigraphically or cyclostratigraphically well constrained marine sections) with volcanic activity; examples are the Triassic-Jurassic boundary and the Central Atlantic Magmatic Province (Schoene et al. 2010), or the lower Toarcian oceanic anoxic event and the Karoo Province volcanism (Sell et al. in prep.). High-precision temporal correlations may also be obtained by combining high-precision U-Pb dating with biochronology in the Middle Triassic (Ovtcharova et al., in prep.), or by comparing U-Pb dates with astronomical timescales in the Upper Miocene (Wotzlaw et al., in prep.). References Guex, J., Schoene, B., Bartolini, A., Spangenberg, J., Schaltegger, U., O'Dogherty, L., et al. (2012). Geochronological constraints on post-extinction recovery of the ammonoids and carbon cycle perturbations during the Early Jurassic. Palaeogeography, Palaeoclimatology, Palaeoecology, 346-347(C), 1-11. Ovtcharova, M., Bucher, H., Schaltegger, U., Galfetti, T., Brayard, A., & Guex, J. (2006). New Early to Middle Triassic U-Pb ages from South China: Calibration with ammonoid biochronozones and implications for the timing of the Triassic biotic recovery. Earth and Planetary Science Letters, 243(3-4), 463-475. Ovtcharova, M., Goudemand, N., Galfetti, Th., Guodun, K., Hammer, O., Schaltegger, U., Bucher, H. Improving accuracy and precision of radio-isotopic and biochronological approaches in dating geological boundaries: The Early-Middle Triassic boundary case. In preparation. Schoene, B., Schaltegger, U., Brack, P., Latkoczy, C., Stracke, A., & Günther, D. (2012). Rates of magma differentiation and emplacement in a ballooning pluton recorded by U-Pb TIMS-TEA, Adamello batholith, Italy. Earth and Planetary Science Letters, 355-356, 162-173. Schoene, B., Latkoczy, C., Schaltegger, U., & Günther, D. (2010). A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMS-TEA). Geochimica Et Cosmochimica Acta, 74(24), 7144-7159. Schoene, B., Guex, J., Bartolini, A., Schaltegger, U., & Blackburn, T. J. (2010). Correlating the end-Triassic mass extinction and flood basalt volcanism at the 100 ka level. Geology, 38(5), 387-390. Sell, B., Ovtcharova, M., Guex, J., Jourdan, F., Schaltegger, U. Evaluating the link between the Karoo LIP and climatic-biologic events of the Toarcian Stage with high-precision U-Pb geochronology. In preparation. Wotzlaw, J. F., Schaltegger, U., Frick, D. A., Dungan, M. A., Gerdes, A., & Günther, D. (2013). Tracking the evolution of large-volume silicic magma reservoirs from assembly to supereruption. Geology, 41(8), 867-870. Wotzlaw, J.F., Hüsing, S.K., Hilgen, F.J.., Schaltegger, U. Testing the gold standard of geochronology against astronomical time: High-precision U-Pb geochronology of orbitally tuned ash beds from the Mediterranean Miocene. In preparation.

Testing the age calibration of the Newark-Hartford APTS by magnetostratigraphic correlation of U-Pb zircon-dated tuffaceous beds in the Late Traissic Chinle Formation in core PFNP-1A from the Petrified Forest National Park (Arizona, USA)

NASA Astrophysics Data System (ADS)

Kent, D. V.; Olsen, P. E.; Mundil, R.; Lepre, C. J.

2017-12-01

The Newark-Hartford APTS extends over 27 Myr according to cycle stratigraphy of the Norian and Rhaetian of the Late Triassic and Hettangian and Sinemurian of the Early Jurassic and an additional 6 Myr by extrapolation into the Carnian; the entire sequence is anchored by U-Pb zircon dating of CAMP activity that provides a calibration date of 201.6 Ma for Chron E23r just below the end-Triassic extinction and the earliest CAMP basalts in the Newark basin (Blackburn+2013 Science; Kent+2017 ESR). The developing APTS has been successfully used for global correlations in marine and non-marine facies but there have been ongoing suggestions that millions of years of Rhaetian time are missing in a cryptic unconformity that supposedly occurs just above E23r in the Newark Supergroup basins. Testing the continuity of the APTS by magnetostratigraphic correlation of U-Pb zircon-dated tuffaceous beds in the Chinle Formation was a prime scientific objective for core PFNP-1A. Paleomagnetic results were obtained using stepwise thermal demagnetization to 680°C from >150 samples of finer-grained red lithologies from the upper 250 m of the cored section of the Chinle (upper Sonsela, Petrified Forest including the Black Forest Bed, and lower Owl Rock Members). Characteristic directions isolated in 2/3 of the samples showed antipodal directions that were shallow with respect to reference directions (flattening factor 0.5), consistent with early acquisition of remanence. Seven polarity magnetozones produce a distinctive pattern correlated to Chrons E17r to E14r of the APTS. The Black Forest Bed at 209.93±0.26 Ma (Ramezani+2011 GSAB), confirmed by our new U-Pb dates from core PFNP-1A, occurs in a reverse polarity magnetozone correlated to E16r (209.95-210.25 Ma), which puts the U-Pb zircon date(s) in excellent agreement with the inferred APTS age. Rather than a 'missing Rhaetian', the apparent regional differences in appearances and disappearances of palynoflora, conchostracans, and other endemic taxa in continental deposits are more likely a reflection of demonstrated continental drift across climate belts and the misinterpretation of ecostratigraphy as chronostratigraphy. A suite of new U-Pb dates in conjunction with paleomagnetic analyses in PFNP-1A is expected to calibrate much of Triassic succession of the Colorado Plateau.

In situ Pb-Pb dating of rutile from slowly cooled granulites by LA-MC-ICP-MS: confirmation of the high closure temperature (>=600°C) for Pb diffusion in rutile

NASA Astrophysics Data System (ADS)

Vry, J.; Baker, J.; Waight, T.

2003-04-01

We have analysed Pb isotopes in natural rutile crystals by laser ablation MC-ICP-MS to assess the potential of rapid Pb-Pb dating of rutile with this method. The rutile samples are from granulite-facies Mg- and Al-rich rocks from the Reynolds Range, Northern Territory, Australia. This metamorphic terrane has a well-constrained high-T cooling history (ca. 3^oC/Myr) defined by previous U-Pb dating of monazite and zircon (peak metamorphism at 1584 Ma), which we have supplemented with additional Rb-Sr dates of phlogopite, biotite and muscovite. The dated rutiles vary in size from 3 to 0.05 mm, have Pb concentrations of ca. 20 ppm, and were analysed with a 266 nm laser coupled to an AXIOM MC-ICP-MS (spot size of 200-50 μm). Individual larger crystals (>= 200 μm) exhibit sufficient Pb isotopic heterogeneity (206Pb/204Pb = 10000-80000) to perform isochron calculations on several short analyses of a single grain (30-60 s). The largest rutiles yielded Pb-Pb isochron ages of 1540-1555 Ma with typical uncertainties of ± 1 to 10 Ma. 207Pb/206Pb ages are typically within 1% of the Pb-Pb isochron ages testifying to the radiogenic nature of Pb in the rutile. A mean age for all the analysed rutiles was 1548.4 ± 9.1 Ma (n = 33). Comparable 207Pb/206Pb ages were also obtained from individual smaller crystals (50 μm) where the 204Pb ion beam could not be measured precisely. The results demonstrate that even small rutile crystals are extremely resistant to isotopic resetting, and that this mineral is a high-T chronometer. Phlogopite and muscovite Rb-Sr ages are <1454 and 1400-1480 Ma, respectively, with some of the phlogopite and biotite micas having been partially reset by later thermal events younger than 400 Ma. All the mica ages are considerably younger (100-70 My) than the rutile ages, which approach U-Pb ages for monazite and zircon overgrowths, even though the mica closure temperatures (350-500^oC) are comparable or slightly higher than earlier geological estimates [1] of the rutile closure temperature. Thus, our results confirm a recent experimental study [2] that suggested the closure temperature for Pb diffusion in rutile (e.g. 100 μm) is much higher (200^oC) than previously thought [1]. [1] Mezger et al., 1989. High precision U-Pb ages of metamorphic rutile: applications to the cooling history of high-grade terranes. EPSL 96, 106-118. [2] Cherniak, 2000. Pb diffusion in rutile. Contrib. Mineral. Petrol., 139, 198-207.

CONCH: A Visual Basic program for interactive processing of ion-microprobe analytical data

NASA Astrophysics Data System (ADS)

Nelson, David R.

2006-11-01

A Visual Basic program for flexible, interactive processing of ion-microprobe data acquired for quantitative trace element, 26Al- 26Mg, 53Mn- 53Cr, 60Fe- 60Ni and U-Th-Pb geochronology applications is described. Default but editable run-tables enable software identification of secondary ion species analyzed and for characterization of the standard used. Counts obtained for each species may be displayed in plots against analysis time and edited interactively. Count outliers can be automatically identified via a set of editable count-rejection criteria and displayed for assessment. Standard analyses are distinguished from Unknowns by matching of the analysis label with a string specified in the Set-up dialog, and processed separately. A generalized routine writes background-corrected count rates, ratios and uncertainties, plus weighted means and uncertainties for Standards and Unknowns, to a spreadsheet that may be saved as a text-delimited file. Specialized routines process trace-element concentration, 26Al- 26Mg, 53Mn- 53Cr, 60Fe- 60Ni, and Th-U disequilibrium analysis types, and U-Th-Pb isotopic data obtained for zircon, titanite, perovskite, monazite, xenotime and baddeleyite. Correction to measured Pb-isotopic, Pb/U and Pb/Th ratios for the presence of common Pb may be made using measured 204Pb counts, or the 207Pb or 208Pb counts following subtraction from these of the radiogenic component. Common-Pb corrections may be made automatically, using a (user-specified) common-Pb isotopic composition appropriate for that on the sample surface, or for that incorporated within the mineral at the time of its crystallization, depending on whether the 204Pb count rate determined for the Unknown is substantially higher than the average 204Pb count rate for all session standards. Pb/U inter-element fractionation corrections are determined using an interactive log e-log e plot of common-Pb corrected 206Pb/ 238U ratios against any nominated fractionation-sensitive species pair (commonly 238U 16O +/ 238U +) for session standards. Also displayed with this plot are calculated Pb/U and Pb/Th calibration line regression slopes, y-intercepts, calibration uncertainties, standard 204Pb- and 208Pb-corrected 207Pb/ 206Pb dates and other parameters useful for assessment of the calibration-line data. Calibrated data for Unknowns may be automatically grouped according to calculated date and displayed in color on interactive Wetherill Concordia, Tera-Wasserburg Concordia, Linearized Gaussian ("Probability Paper") and Gaussian-summation probability density diagrams.

Using zircon (U-Th)/He damage-diffusivity patterns to quantify detachment-related basement exhumation in the Mecca Hills, CA

NASA Astrophysics Data System (ADS)

Moser, A. C.; Ault, A. K.; Evans, J. P.; Reiners, P. W.; Stearns, M.; Guenthner, W.

2017-12-01

Exposures of gneiss and Orocopia Schist (OS) in the Mecca Hills, California, adjacent to the southernmost San Andreas Fault system, preserve the exhumation history of Oligocene detachment faulting. We investigate the duration, magnitude, and mechanisms of exhumation of these units at regional and local scales using in situ U-Pb zircon dating (n = 248), (U-Th)/He (He) thermochronometry (n = 39), and He date-effective U (eU) patterns. Zircons with variable preserved visual metamictization were targeted for He analyses to purposefully build a dataset with a range in eU concentration and zircon He closure temperatures, as well as induce a He date-eU correlation. Analyzed zircon crystals range from clear and transparent to purple-brown and translucent in each sample. Zircon cathodoluminescence images reveal oscillatory and sector chemical zoning. Each sample contains a population of largely Proterozoic U-Pb dates implying some grains accumulated radiation damage since 1.9-1.1 Ga. Zircon (U-Th)/He dates from seven samples of OS and gneiss yield a mean date of 24 ± 3.5 Ma (n = 32) and uniform dates over an 90-2950 ppm eU range. One gneiss sample yields a mean date of 65 ± 5.6 Ma (n = 7) over a limited eU spread ( 500-950 ppm). Mean zircon He dates from these two units overlap, but dates are broadly younger in northeastern exposures dominated by OS. Preliminary thermal history simulations integrating zircon U-Pb data, He date-eU patterns, and independent geologic constraints require at least 200 °C of cooling through the zircon He partial retention zone 30-21 Ma and show that the pre-70 Ma thermal history does not affect the predicted date-eU correlation. This shared rapid cooling history documented in the OS and gneiss imply these units were juxtaposed prior to 30 Ma and exhumed as a coherent structural block within the footwall of the Orocopia Mountains Detachment Fault in the Mecca Hills. Spatio-temporal variation in mean zircon He dates may delineate time-transgressive cooling of these units as they exhumed. Zircon textures and overall invariant He dates regardless of eU imply that visual metamictization persists in zircons at temperatures >200 °C and indicates these grains experienced a thermal history characterized by temperatures that preserved visual damage but fully induced He loss in the crystals prior to exhumation.

U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite melange, southwest Japan: insights into the timing of serpentinization

USGS Publications Warehouse

Tsujimori, T.; Liou, J.G.; Wooden, J.; Miyamoto, T.

2005-01-01

Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite mélange, Southwest Japan. The zircon porphyroblasts show pronounced zoning, and are characterized by both low Th/U ratios (0.2-0.8) and low Th and U abundances (Th = 1-81 ppm; U = 6-149 ppm). They contain inclusions of high-pressure minerals, including jadeite and rutile; such an occurrence indicates that the zircon crystallized during subduction-zone metamorphism. Phase equilibria and the existing fluid-inclusion data constrain P-T conditions to P > 1.2 GPa at T > 350°C for formation of the jadeitite. Most U/Pb ages obtained by SHRIMP-RG are concordant, with a weighted mean 206Pb/238U age of 472 ± 8.5 Ma (MSWD = 2.7, n = 25). Because zircon porphyroblasts contain inclusions of high-pressure minerals, the SHRIMP U-Pb age represents the timing of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction zone. Although this dating does not provide a direct time constraint for serpentinization, U-Pb ages of zircon in jadeitite associated with serpentinite result in new insights into the timing of fluid-rock interaction of ultramafic rocks at a subduction zone and the minimum age for serpentinization.

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

NASA Astrophysics Data System (ADS)

Vazquez, J. A.; Matthews, N. E.; Calvert, A. T.

2015-12-01

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff (LCT). Tephra from the eruption blanketed much of the western United States, and is a key Quaternary chronostratigraphic marker, in particular for dating deposition of mid-Pleistocene glacial and pluvial deposits in western North America. We performed 40Ar/39Ar dating of single sanidines to delimit eruption age, and ion microprobe U-Pb and trace-element analyses of crystal faces on single zircons to characterize magmatic evolution and date near-eruption crystallization, as well as analyses of crystal interiors to date the interval of zircon crystallization. Sanidines from the two LCT members A and B yield an 40Ar/39Ar isochron date of 631 ± 4 ka (2σ). Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 627 ± 6 ka (2σ) and have trace element concentrations that vary with eruptive stratigraphy. Zircon interiors yield a weighted mean 206Pb/238U date of 660 ± 6 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high-U concentrations and dark cathodoluminescence (CL) cores. These crystals with high-U cores are possibly sourced from 'defrosting' of melt-impregnated margins of the growing subvolcanic reservoir. LCT sanidines mirror the variation of zircon composition within the eruptive stratigraphy, with crystals from upper LCT-A and basal LCT-B having bright-CL rims with high Ba concentrations, suggesting late crystallization after addition of less evolved silicic magma. The occurrence of distal LCT in stratigraphic sequences marking the Marine Isotope Stage 16-15 transition supports the apparent eruption age of ca. 631 ka. These results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103-104 year interval before eruption.

High spatial resolution U-Pb geochronology and Pb isotope geochemistry of magnetite-apatite ore from the Pea Ridge iron oxide-apatite deposit, St. Francois Mountains, southeast Missouri, USA

USGS Publications Warehouse

Neymark, Leonid; Holm-Denoma, Christopher S.; Pietruszka, Aaron; Aleinikoff, John N.; Fanning, C. Mark; Pillers, Renee M.; Moscati, Richard J.

2016-01-01

The Pea Ridge iron oxide-apatite (IOA) deposit is one of the major rhyolite-hosted magnetite deposits of the St. Francois Mountains terrane, which is located within the Mesoproterozoic (1.5–1.3 Ga) Granite-Rhyolite province in the U.S. Midcontinent. Precise and accurate determination of the timing and duration of oreforming processes in this deposit is crucial for understanding its origin and placing it within a deposit-scale and regional geologic context. Apatite and monazite, well-established U-Pb mineral geochronometers, are abundant in the Pea Ridge orebody. However, the potential presence of multiple generations of dateable minerals, processes of dissolution-reprecipitation, and occurrence of micrometer-sized intergrowths and inclusions complicate measurements and interpretations of the geochronological results. Here, we employ a combination of several techniques, including ID-TIMS and high spatial resolution geochronology of apatite and monazite using LA-SC-ICPMS and SHRIMP, and Pb isotope geochemistry of pyrite and magnetite to obtain the first direct age constraints on the formation and alteration history of the Pea Ridge IOA deposit. The oldest apatite TIMS 207Pb*/206Pb* dates are 1471 ± 1 and 1468 ± 1 Ma, slightly younger than (but within error of) the ~1474 to ~1473 Ma U-Pb zircon ages of the host rhyolites. Dating of apatite and monazite inclusions within apatite provides evidence for at least one younger metasomatic event at ~1.44 Ga, and possibly multiple superimposed metasomatic events between 1.47 and 1.44 Ga. Lead isotop analyses of pyrite show extremely radiogenic 206Pb/204Pb ratios up to ~80 unsupported by in situ U decay. This excess radiogenic Pb in pyrite may have been derived from the spatially associated apatite as apatite recrystallized several tens of million years after its formation. The low initial 206Pb/204Pb ratio of ~16.5 and 207Pb/204Pb ratio of ~15.4 for individual magnetite grains indicate closed U-Pb system behavior in this mineral and are consistent with derivation of the Pb from a mantle-like source.

U Pb ages of angrites

NASA Astrophysics Data System (ADS)

Amelin, Yuri

2008-01-01

Precise U-Pb ages, determined with double spike ( 202Pb- 205Pb) thermal ionization m1ass spectrometry, are reported for angrites Angra dos Reis (AdoR), Lewis Cliff 86010 (LEW), and D'Orbigny. Nineteen of 23 acid-washed pyroxene fractions from these meteorites and whole rock fractions from D'Orbigny contain between 0.5 and 1.3 pg of total common Pb, indistinguishable from analytical blank. Measured 206Pb/ 204Pb ratios in these fractions are between 6300 and 14,100 for AdoR, 1160-4500 for LEW, and 608-8500 for D'Orbigny. Blank-corrected 206Pb/ 204Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible 207Pb ∗/ 206Pb ∗ dates with the average values of 4557.65 ± 0.13 Ma for AdoR, 4558.55 ± 0.15 Ma for LEW, and 4564.42 ± 0.12 Ma for D'Orbigny. Pb-Pb isochrons including data with slightly elevated common Pb, and U-Pb upper concordia intercepts, yield similar dates. The implications of these new Pb-isotopic ages of angrites are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of "slowly cooled" angrites is therefore genetically diverse. Second, the new age of LEW suggests an upward revision of 53Mn- 53Cr "absolute" ages by 0.7 Ma. Third, a precise age of D'Orbigny allows consistent linking of the 53Mn- 53Cr and 26Al- 26Mg extinct nuclide chronometers to the absolute lime scale.

U-Th-Pb zircon dating of the 13.8-Ma dacite volcanic dome at Cerro Rico de Potosi, Bolivia

USGS Publications Warehouse

Zartman, R.E.; Cunningham, C.G.

1995-01-01

The temporal relationship between the extrusion of the Miocene dacite volcanic dome at Cerro Rico de Potasi, Bolivia, and the associated Ag-Sn mineralization has an important bearing on the heat and metal sources for this world class mineral deposit. The present study uses U-Th-Pb dating of sparse zircon contained in the dacite to demonstrate that, at most, only several hundred thousand years separate dome emplacement from main stage mineralization. -from Authors

Hydrothermal titanite from the Chengchao iron skarn deposit: temporal constraints on iron mineralization, and its potential as a reference material for titanite U-Pb dating

NASA Astrophysics Data System (ADS)

Hu, Hao; Li, Jian-Wei; McFarlane, Christopher R. M.

2017-09-01

Uranium-lead isotopes and trace elements of titanite from the Chengchao iron skarn deposit (Daye district, Eastern China), located along the contact zones between Triassic marine carbonates and an early Cretaceous intrusive complex consisting of granite and quartz diorite, were analyzed using laser ablation inductively coupled plasma mass spectrometry to provide temporal constraints on iron mineralization and to evaluate its potential as a reference material for titanite U-Pb geochronology. Titanite grains from mineralized endoskarn have simple growth zoning patterns, exhibit intergrowth with magnetite, diopside, K-feldspar, albite and actinolite, and typically contain abundant primary two-phase fluid inclusions. These paragenetic and textural features suggest that these titanite grains are of hydrothermal origin. Hydrothermal titanite is distinct from the magmatic variety from the ore-related granitic intrusion in that it contains unusually high concentrations of U (up to 2995 ppm), low levels of Th (12.5-453 ppm), and virtually no common Pb. The REE concentrations are much lower, as are the Th/U and Lu/Hf ratios. The hydrothermal titanite grains yield reproducible uncorrected U-Pb ages ranging from 129.7 ± 0.7 to 132.1 ± 2.7 Ma (2σ), with a weighted mean of 131.2 ± 0.2 Ma [mean standard weighted deviation (MSWD) = 1.7] that is interpreted as the timing of iron skarn mineralization. This age closely corresponds to the zircon U-Pb age of 130.9 ± 0.7 Ma (MSWD = 0.7) determined for the quartz diorite, and the U-Pb ages for zircon and titanite (130.1 ± 1.0 Ma and 131.3 ± 0.3 Ma) in the granite, confirming a close temporal and likely genetic relationship between granitic magmatism and iron mineralization. Different hydrothermal titanite grains have virtually identical uncorrected U-Pb ratios suggestive of negligible common Pb in the mineral. The homogeneous textures and U-Pb characteristics of Chengchao hydrothermal titanite suggest that the mineral might be a suitable internal reference material for U-Pb dating.

Capability of U-Pb dating of zircons from Quaternary tephra: Jemez Mountains, NM, and La Sal Mountains, UT, USA

NASA Astrophysics Data System (ADS)

Krautz, Jana; Hofmann, Mandy; Gärtner, Andreas; Linnemann, Ulf; Kleber, Arno

2018-01-01

Two Quaternary tephras derived from the Jemez Mountains, New Mexico - the Guaje and Tsankawi tephras - are difficult to distinguish due to their similar glass-shard chemical composition. Differences in bulk chemical composition are small as well. Here we examine the feasibility to assign an age to a distal tephra layer in the La Sal Mountains, Utah, by U-Pb dating of zircons and to correlate it with one of the two Jemez eruptions. We also dated original Jemez tephras for comparison. Even though the tephras are very young, we obtained reasonable age determinations using the youngest cluster of zircon grains overlapping in age at 2σ. Thereafter, the Guaje tephra is 1.513 ± 0.021 Myr old. The La Sal Mountains tephra is correlated with the Tsankawi tephra. Three samples yielded a common age range of 1.31-1.40 Myr. All ages are in slight disagreement with published age determinations obtained by 40Ar / 39Ar dating. These findings indicate that distal Jemez tephras can be distinguished by U-Pb dating. Furthermore, we encourage giving this method a try for age assignments even of Quaternary volcanic material.

Cr-rich rutile: A powerful tool for diamond exploration

NASA Astrophysics Data System (ADS)

Malkovets, V. G.; Rezvukhin, D. I.; Belousova, E. A.; Griffin, W. L.; Sharygin, I. S.; Tretiakova, I. G.; Gibsher, A. A.; O'Reilly, S. Y.; Kuzmin, D. V.; Litasov, K. D.; Logvinova, A. M.; Pokhilenko, N. P.; Sobolev, N. V.

2016-11-01

Mineralogical studies and U-Pb dating have been carried out on rutile included in peridotitic and eclogitic garnets from the Internatsionalnaya pipe, Mirny field, Siberian craton. We also describe a unique peridotitic paragenesis (rutile + forsterite + enstatite + Cr-diopside + Cr-pyrope) preserved in diamond from the Mir pipe, Mirny field. Compositions of rutile from the heavy mineral concentrates of the Internatsionalnaya pipe and rutile inclusions in crustal almandine-rich garnets from the Mayskaya pipe (Nakyn field), as well as from a range of different lithologies, are presented for comparison. Rutile from cratonic mantle peridotites shows characteristic enrichment in Cr, in contrast to lower-Cr rutile from crustal rocks and off-craton mantle. Rutile with Cr2O3 > 1.7 wt% is commonly derived from cratonic mantle, while rutiles with lower Cr2O3 may be both of cratonic and off-cratonic origin. New analytical developments and availability of standards have made rutile accessible to in situ U-Pb dating by laser ablation ICP-MS. A U-Pb age of 369 ± 10 Ma for 9 rutile grains in 6 garnets from the Internatsionalnaya pipe is consistent with the accepted eruption age of the pipe (360 Ma). The equilibrium temperatures of pyropes with rutile inclusions calculated using Ni-in-Gar thermometer range between 725 and 1030 °C, corresponding to a depth range of ca 100-165 km. At the time of entrainment in the kimberlite, garnets with Cr-rich rutile inclusions resided at temperatures well above the closure temperature for Pb in rutile, and thus U-Pb ages on mantle-derived rutile most likely record the emplacement age of the kimberlites. The synthesis of distinctive rutile compositions and U-Pb dating opens new perspectives for using rutile in diamond exploration in cratonic areas.

U, Th, Pb and REE abundances and Pb 207/Pb 206 ages of individual minerals in returned lunar material by ion microprobe mass analysis.

NASA Technical Reports Server (NTRS)

Andersen, C. A.; Hinthorne, J. R.

1972-01-01

Results of ion microprobe analyses of Apollo 11, 12 and 14 material, showing that U, Th, Pb and REE are concentrated in accessory minerals such as apatite, whitlockite, zircon, baddeleyite, zirkelite, and tranquillityite. Th/U ratios are found to vary by over a factor of 40 in these minerals. K, Ba, Rb and Sr have been localized in a K rich, U and Th poor glass phase that is commonly associated with the U and Th bearing accessory minerals. Li is observed to be fairly evenly distributed between the various accessory phases. The phosphates have been found to have REE abundance patterns (normalized to the chondrite abundances) that are fairly flat, while the Zr bearing minerals have patterns that rise steeply, by factors of ten or more, from La to Gd. All the accessory minerals have large negative Eu anomalies. Radiometric age dates (Pb 207/Pb 206) of the individual U and Th bearing minerals compare favorably with the Pb 207/Pb 206 age of the bulk rocks.

Development of a petrochronology laboratory in Russia

NASA Astrophysics Data System (ADS)

Yavryan, M.; Powerman, V.; Nourgaliev, D. K.

2017-12-01

Each year more and more U-Pb detrital zircon studies are coming from Russia. However, the latest developments in zircon studies, namely combining U-Pb age with the measurements of REE concentrations and Lu-Hf isotopy on the same grain, have been unavailable in Russia. Kazan Federal U. has undertaken certain efforts in order to develop Russia's first petrochronology laboratory. The following equipment was recently purchased, installed and put into operation: Neptune Plus HR MC ICPMS, with jet-interface and RPQ filter. iCAP-Qc quadruple ICPMS. 2 x NWR213 laser ablation systems; one of them equipped with the TwoVol2 ablation chamber. Helium gas is used to flush the ablation chamber; 5 ml/min. of nitrogen is mixed downstream before plasma in order to increase the signal. The following methods have been set up at KFU on the Neptune Plus ICPMS: (1) U-Pb geochronology on zircons. A set of standards have been dated (R33, 91500, Plesovice, Mudtank, GJ-1, AS-3); (2) U-Pb geochronology on perovskites. We have collected, separated and analyzed Baikal Tazheran 463 Ma perovskite, previously dated on TIMS (e.g., Ireland et al., 1990; Li et al., 2010), using 91500 zircon as a primary standard. We intend to start using Tazheran perovskite as an in-house standard for dating perovskites. (3) Lu-Hf isotopy on a set of standard zircons. All results will be presented during the poster session. Our next steps will include setting the (a) U-Pb geochronology and (b) REE measurements on the quadruple MS; (c) splitting the ablation stream into two and directing them to Neptune for Lu-Hf measurements and to iCAP for either U-Pb, or REE, or both types of measurements; (d) experimenting with pre-ablation in order to diminish common lead, with (e) rastering during ablation to diminish downhole fractionation, with (f) squid signal smoothing device.

An algorithm for U-Pb isotope dilution data reduction and uncertainty propagation

NASA Astrophysics Data System (ADS)

McLean, N. M.; Bowring, J. F.; Bowring, S. A.

2011-06-01

High-precision U-Pb geochronology by isotope dilution-thermal ionization mass spectrometry is integral to a variety of Earth science disciplines, but its ultimate resolving power is quantified by the uncertainties of calculated U-Pb dates. As analytical techniques have advanced, formerly small sources of uncertainty are increasingly important, and thus previous simplifications for data reduction and uncertainty propagation are no longer valid. Although notable previous efforts have treated propagation of correlated uncertainties for the U-Pb system, the equations, uncertainties, and correlations have been limited in number and subject to simplification during propagation through intermediary calculations. We derive and present a transparent U-Pb data reduction algorithm that transforms raw isotopic data and measured or assumed laboratory parameters into the isotopic ratios and dates geochronologists interpret without making assumptions about the relative size of sample components. To propagate uncertainties and their correlations, we describe, in detail, a linear algebraic algorithm that incorporates all input uncertainties and correlations without limiting or simplifying covariance terms to propagate them though intermediate calculations. Finally, a weighted mean algorithm is presented that utilizes matrix elements from the uncertainty propagation algorithm to propagate random and systematic uncertainties for data comparison between other U-Pb labs and other geochronometers. The linear uncertainty propagation algorithms are verified with Monte Carlo simulations of several typical analyses. We propose that our algorithms be considered by the community for implementation to improve the collaborative science envisioned by the EARTHTIME initiative.

206Pb-230Th-234U-238U and 207Pb-235U geochronology of Quaternary opal, Yucca Mountain, Nevada

USGS Publications Warehouse

Neymark, Leonid A.; Amelin, Yuri V.; Paces, James B.

2000-01-01

U–Th–Pb isotopic systems have been studied in submillimeter-thick outermost layers of Quaternary opal occurring in calcite–silica fracture and cavity coatings within Tertiary tuffs at Yucca Mountain, Nevada, USA. These coatings preserve a record of paleohydrologic conditions at this site, which is being evaluated as a potential high-level nuclear waste repository. The opal precipitated from groundwater is variably enriched in 234U (measured 234U/238U activity ratio 1.124–6.179) and has high U (30–313 ppm), low Th (0.008–3.7 ppm), and low common Pb concentrations (measured 206Pb/204Pb up to 11,370). It has been demonstrated that the laboratory acid treatment used in this study to clean sample surfaces and to remove adherent calcite, did not disturb U–Th–Pb isotopic systems in opal. The opal ages calculated from 206Pb∗/238U and 207Pb∗/235U ratios display strong reverse discordance because of excess radiogenic 206Pb∗ derived from the elevated initial 234U. The data are best interpreted using projections of a new four-dimensional concordia diagram defined by 206Pb∗/238U, 207Pb∗/235U, 234U/238Uactivity, and 230Th/238Uactivity. Ages and initial 234U/238U activity ratios have been calculated using different projections of this diagram and tested for concordance. The data are discordant, that is observed 207Pb∗/235U ages of 170 ± 32 (2σ) to 1772 ± 40 ka are systematically older than 230Th/U ages of 34.1 ± 0.6 to 452 ± 32 ka. The age discordance is not a result of migration of uranium and its decay products under the open system conditions, but a consequence of noninstantaneous growth of opal. Combined U–Pb and 230Th/U ages support the model of slow mineral deposition at the rates of millimeters per million years resulting in layering on a scale too fine for mechanical sampling. In this case, U–Pb ages provide more accurate estimates of the average age for mixed multiage samples than 230Th/U ages, because ages based on shorter-lived isotopes are nonlinearly biased by younger mineral additions. Use of the combined U–Th–Pb technique to date Yucca Mountain Quaternary opals significantly extends the age range beyond that of the 230Th/U dating method and shows that selected fracture pathways in the unsaturated zone felsic tuffs of Yucca Mountain have been active throughout the Quaternary.

Datation U_Pb : âge de mise en place du magmatisme bimodal des Jebilet centrales (chaîne Varisque, Maroc). Implications géodynamiquesU_Pb dating: emplacement age of the bimodal magmatism of central Jebilet (Variscan Belt, Morocco). Geodynamic implications

NASA Astrophysics Data System (ADS)

Essaifi, Abderrahim; Potrel, Alain; Capdevila, Ramon; Lagarde, Jean-Louis

2003-01-01

The bimodal magmatism of central Jebilet is dated to 330.5 +0.68-0.83 Ma by UPb dating on zircons. This age, similar to that of the syntectonic Jebilet cordierite-bearing granitoids, corresponds to the age of the local major tectonometamorphic event. The syntectonic plutonism of the Jebilet massif, composed of tholeiitic, alkaline, and peraluminous calc-alkaline series, is variegated. Magmas emplacement was favoured by the local extension induced by the motion along the western boundary of the Carboniferous basins of the Moroccan Meseta. The Jebilet massif exemplifies the activation of various magmas sources during an episode of continental convergence and crustal wrenching.

Correlating rates of magmatic arc unroofing and sedimentation using detrital zircon U/Pb and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Fosdick, J. C.

2017-12-01

Double and triple dating of minerals using multiple geo-thermochronometers has revolutionized efforts to evaluate complex thermotectonic histories of orogens, isolate unique sedimentary sources, and quantify basin burial reheating. A persisting challenge is to distinguish volcanic sources from rapidly exhumed sources, with the simplistic premise that coincident cooling dates among high- to low-temperature thermochronometers are diagnostic of volcanic sources. Coupled zircon U/Pb and (U-Th)/He geo-thermochronometry from the Miocene Bermejo foreland basin in the southern Central Andes reveals a high temporal resolution of unroofing signatures of the Choiyoi Group, a Permian-Triassic silicic volcanic and plutonic complex, and the Pennsylvanian-Permian Colangüil batholith. Both units are important sediment sources within the High Andes for the Cenozoic east-flowing sediment routing systems. Results show fluvial sourcing of Colangüil detrital zircons with progressively greater partial loss of He (<8% to 12-23% fractional loss from 9.5 Ma to 6 Ma), as indicated by upsection younging of zircon He dates for a given U/Pb age cluster. These findings suggest erosion of increasingly deeper levels of the Colangüil arc during late Miocene development of the High Andes. This progression of higher He loss and thus younger He dates during sedimentation for a given U/Pb age cluster is analogous to the magmatic arc unroofing trend revealed by undissected to dissected arc provenance fields in sandstone petrography. Multi-method thermochronometry of detrital minerals may reveal an added level of information regarding rates of cooling, unroofing, and thermal evolution of magmatic systems as preserved in the detrital record.

U-Pb detrital zircon geochronology from the basement of the Central Qilian Terrane: implications for tectonic evolution of northeastern Tibetan Plateau

NASA Astrophysics Data System (ADS)

Liu, Changfeng; Wu, Chen; Zhou, Zhiguang; Yan, Zhu; Jiang, Tian; Song, Zhijie; Liu, Wencan; Yang, Xin; Zhang, Hongyuan

2018-03-01

The Tuolai Group dominates the Central Qilian Terrane, and there are different opinions on the age and tectonic attribute of the Tuolai Group. Based on large-scale geologic mapping and zircon dating, the Tuolai Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusive rocks, early-middle Ordovician acid intrusive rocks and middle Ordovician basic intrusive rocks. The metamorphic supracrustal rocks are the redefined Tuolai complex-group and include gneiss and schist assemblage by faulting contact. Zircon U-Pb LA-MC-ICP-MS dating was conducted on these samples of gneiss and migmatite from the gneiss assemblage, quartzite, two-mica schist and slate from the schist assemblage. The five detrital samples possess similar age spectra; have detrital zircon U-Pb main peak ages of 1.7 Ga with youngest U-Pb ages of 1150 Ma. They are intruded by Neoproterozoic acid intrusive rocks. Therefore, the Tuolai Group belonging to late Mesoproterozoic and early Neoproterozoic. With this caveat in mind, we believe that U-Pb detrital zircon dating, together with the geologic constraints obtained from this study and early work in the neighboring regions. We suggest that the formation age of the entire crystalline basement rocks of metasedimentary sequence from the Central Qilian Terrane should be constrained between the Late Mesoproterozoic and the Late Neoproterozoic, but not the previous Paleoproterozoic. The basement of the Central Qilian Terrane contains the typical Grenville ages, which indicates the Centre Qilian Terrane have been experienced the Grenville orogeny event.

In-Situ U-Pb Dating of Apatite by Hiroshima-SHRIMP: Contributions to Earth and Planetary Science.

PubMed

Terada, Kentaro; Sano, Yuji

2012-01-01

The Sensitive High Resolution Ion MicroProbe (SHRIMP) is the first ion microprobe dedicated to geological isotopic analyses, especially in-situ analyses related to the geochronology of zircon. Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda (1974), was constructed at the Australian National University. In 1996, such an instrument was installed at Hiroshima University and was the first SHRIMP to be installed in Japan. Since its installation, our focus has been on the in-situ U-Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. This provides the possibility for extending the use of in-situ U-Pb dating from determining the age of formation of volcanic, granitic, sedimentary and metamorphic minerals to the direct determination of the diagenetic age of fossils and/or the crystallization age of various meteorites, which can provide new insights into the thermal history on the Earth and/or the Solar System. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years.

High-precision Temporal Calibration of the Early Cambrian Biotic and Paleoenvironmental Records: New U-Pb Geochronology from Eastern Yunnan, China.

NASA Astrophysics Data System (ADS)

Tsukui, K.; Ramezani, J.; Zhu, M.; Maloof, A. C.; Porter, S.; Moore, J.; Eddy, M. P.; Bowring, S. A.

2016-12-01

The Terreneuvian Epoch of the early Cambrian marks the global diversification of early animal life, as well as major perturbations to Earth's geochemical cycles. Understanding possible links between biotic evolution (e.g., emergence of skeletal animals) and the recognized changes in ocean chemistry requires a high fidelity chronostratigraphic framework for the early Cambrian records. One such chronostratigraphy was built by mapping local early Cambrian carbon isotope profiles onto a U-Pb age-calibrated marine carbonate δ13C record from Morocco, assuming global synchroneity of the observed δ13C trends. Here we present a direct test of this assumption using high-precision U-Pb geochronology (CA-ID-TIMS method) of ash beds from key lower Cambrian horizons throughout eastern Yunnan Province in South China. Preliminary age results from ash beds near the top of the Dengying Formation (Fm.) and the basal Daibu Member (Mb.) of the Zhujiaqing Fm. in multiple sections place the basal Cambrian negative δ13C excursion (BACE) in China at ca. 540.7-539.6 Ma. Our new U-Pb dates from the overlying Zhongyicun Mb. at the Meishucun and nearby sections improve significantly upon previous in situ U-Pb geochronology and constrain the onset of high-frequency δ13C oscillations in some sections to between 533.5 and 532.9 Ma. Most importantly, a new U-Pb date of ca. 526 Ma from the basal Shiyantou Fm. in the Xiaotan Section marks the termination of a >1 million year-long period of consistently positive (≥+4‰) δ13C values (ZHUCE) that is characteristic of many early Cambrian records worldwide. This date establishes a robust time correlation between ZHUCE in South China and its equivalent 5p excursion in Morocco and Siberia, and constrains the timing and duration of the largest positive δ13C anomalies in the Cambrian.

New uppermost Cambrian U-Pb date from Avalonian Wales and age of the Cambrian-Ordovician boundary

USGS Publications Warehouse

Davidek, K.; Landing, E.; Bowring, S.A.; Westrop, S.R.; Rushton, A.W.A.; Fortey, R.A.; Adrain, J.M.

1998-01-01

A crystal-rich volcaniclastic sandatone in the lower Peltura scarabaeoides Zone at Ogof-odi near Criccieth, North Wales, yields a U-Pb zircon age of 491 ?? 1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian-Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparble to that provided by Ordovician graptolites and Mesozoic ammonites.

From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)

NASA Astrophysics Data System (ADS)

Paquette, J.-L.; Ballèvre, M.; Peucat, J.-J.; Cornen, G.

2017-12-01

In the Variscan belt of Western Europe, the lifetime and evolution of the oceanic domain is poorly constrained by sparse, outdated and unreliable multigrain ID-TIMS U-Pb zircon dating. In this article, we present a complete in situ LA-ICP-MS dataset of about 300 U-Pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of Southern Brittany, comprising new dating of previously published zircon populations and newly discovered rock samples. In situ dating and cathodo-luminescence imaging of each zircon grain yields new absolute time-constraints on the evolution of the Galicia-Moldanubian Ocean. The new results confirm that the opening of this oceanic domain is well defined at about 490 Ma. In contrast, the generally-quoted 400-410 Ma-age for the high-pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites. In light of these new data, we propose that the obduction of oceanic rocks occurred at about 370-380 Ma while the high-pressure event is recorded at 355 Ma in only a few zircon grains of some eclogite samples. Additionally, this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their U-Pb systems do not record that metamorphism systematically. These zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith. Another example of an eclogite sample from the French Massif Central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism.

Magmatic Longevity Constrained by ID-TIMS U-Pb Dating of Zircon and Titanite

NASA Astrophysics Data System (ADS)

Szymanowski, D.; Wotzlaw, J. F.; Ellis, B. S.; Bachmann, O.; Von Quadt, A.

2016-12-01

Clues about the timescales and thermal conditions associated with the growth and evacuation of large silicic magma reservoirs are frequently drawn from radiometric dating, diffusion modelling, or thermomechanical modelling. A growing amount of petrological and geochronological evidence, supported by thermal modelling, suggests that many silicic magma reservoirs may exist for some 104-106 years in the form of high-crystallinity mushes at relatively low temperatures ( 700-750°C; [1-3]). Geochronological studies addressing this issue typically utilise the U-Pb system in zircon capable of recording extended periods of crystallisation, particularly in evolved calc-alkaline systems that spend most of their lifetime zircon-saturated. In this study, we integrate U-Pb dating of zircon and titanite to investigate the longevity of the magma reservoir that produced the Kneeling Nun Tuff, a 35 Ma, >900 km3 crystal-rich rhyolitic super-eruption from the Mogollon-Datil volcanic field in New Mexico (USA). High-precision ID-TIMS U-Pb dates of single crystals of both zircon and titanite independently record a continuous crystallisation history over >400,000 years. We combine the dating of both accessory phases with textural, major, trace element and isotopic studies of single crystals, placing tight constraints on the thermal conditions of magma accumulation and storage while recording differentiation and rejuvenation processes within the magma reservoir. The results suggest a protracted `cool' upper-crustal storage of magma prior to the Kneeling Nun Tuff eruption followed by a melting event which reduced the magma crystallinity and conditioned it for eruption. [1] Bachmann & Bergantz (2004), J. Petrol. 45, 1565-1582. [2] Gelman et al. (2013), Geology 41, 759-762. [3] Cooper & Kent (2014), Nature 506, 480-483.

U-Pb Ages of Lunar Apatites

NASA Technical Reports Server (NTRS)

Vaughan, J.; Nemchin, A. A.; Pidgeon, R. T.; Meyer, Charles

2006-01-01

Apatite is one of the minerals that is rarely utilized in U-Pb geochronology, compared to some other U-rich accessory phases. Relatively low U concentration, commonly high proportion of common Pb and low closure temperature of U-Pb system of apatite inhibit its application as geochronological tool when other minerals such as zircon are widely available. However, zircon appear to be restricted to certain type of lunar rocks, carrying so called KREEP signature, whereas apatite (and whitlockite) is a common accessory mineral in the lunar samples. Therefore, utilizing apatite for lunar chronology may increase the pool of rocks that are available for U-Pb dating. The low stability of U-Pb systematics of apatite may also result in the resetting of the system during meteoritic bombardment, in which case apatite may provide an additional tool for the study of the impact history of the Moon. In order to investigate these possibilities, we have analysed apatites and zircons from two breccia samples collected during the Apollo 14 mission. Both samples were collected within the Fra Mauro formation, which is interpreted as a material ejected during the impact that formed the Imbrium Basin.

A Uranium-Lead Chronology of Speleothem Deposition in the Canadian Arctic

NASA Astrophysics Data System (ADS)

Gambino, C.; Shakun, J. D.; McGee, D.; Ramezani, J.; Khadivi, S.; Wong, C. I.

2017-12-01

The Artic is one of the fastest warming regions on the planet. Currently much of the Arctic is covered by permafrost, which contains approximately 1,700 gigatons of organic carbon. Permafrost thaw could release a substantial amount of this carbon as greenhouse gases into the atmosphere through microbial decomposition, potentially dramatically amplifying anthropogenic warming. However, the risk of permafrost thaw is uncertain, with models exhibiting a wide range of possibilities. Assessing the stability of permafrost during past interglacial periods enables evaluation of the sensitivity of permafrost to warming. Cave mineral deposits (speleothems) in areas currently covered with permafrost can act as a proxy for past permafrost thaw, as liquid water is one criteria of speleothem growth and thus implies thawed ground conditions. Previous uranium-thorium (U-Th) dating of speleothems (n=67) from a wide range of latitudes and permafrost zones across the southern Canadian Rockies, Northwest Territories, and the northern Yukon suggest deposition during Marine Isotope Stage (MIS) 11 and 13. The majority of U-Th dates of these speleothems, however, exceed the U-Th dating limit of 600 ka. In this study, we apply uranium-lead (U-Pb) geochronology to several of these speleothems to extend the records of speleothem growth further back in time. Initial results include a U-Pb age of 428 ± 14 ka that replicates a previous U-Th age of 416.8 ± 7.9 ka, and U-Pb ages on two other speleothems of 870 ± 100 ka and 1502 ± 30 ka. The results of currently in progress U-Pb analyses and a comparison of results with paleo-temperature and ice volume reconstructions will also be presented.

U-Pb and Lu-Hf zircon geochronology of the Cañadón Asfalto Basin, Chubut, Argentina: Implications for the magmatic evolution in central Patagonia

NASA Astrophysics Data System (ADS)

Hauser, N.; Cabaleri, N. G.; Gallego, O. F.; Monferran, M. D.; Silva Nieto, D.; Armella, C.; Matteini, M.; Aparicio González, P. A.; Pimentel, M. M.; Volkheimer, W.; Reimold, W. U.

2017-10-01

The Cañadón Asfalto basin, central Chubut, Argentina, comprises a volcano-sedimentary sequence related to the opening of the Atlantic Ocean during Mesozoic times. The Lonco Trapial, Cañadón Asfalto and Cañadón Calcáreo formations are the main units related to the evolution of this basin. The Las Chacritas and Puesto Almada members are distinguished in the Cañadón Asfalto Formation. LA-HR-ICP-MS U-Pb and Lu-Hf data on zircon were obtained on these units. The Lonco Trapial Formation gave a weighted average age of 172.3 ± 1.8 Ma. A pyroclastic level from the Las Chacritas Member gave a weighted average age of 168.2 ± 2.2 Ma. Two U-Pb concordant ages of 160.3 ± 1.7 Ma on a laminated tuffite and 158.3 ± 1.3 Ma on a pyroclastic level were obtained for the Puesto Almada Member. Two maximum depositional ages constrain the sedimentary provenance areas for the basin: 1) A sample from the Sierra de la Manea range, where a controversial unit related either to the Cañadón Asfalto or to the Cañadón Calcáreo formation occurs, gave an age of 176.6 ± 1.0 Ma. Two younger zircon crystals indicate that this unit may be related to the Cañadón Calcáreo Formation. 2) A sandstone with cross-stratification from the Puesto Almada Member gave a maximum depositional age of 173.6 ± 6.4 Ma. In terms of U-Pb and Lu-Hf isotopes, two magmatic events are identified in central Patagonia: the Mamil Choique magmatic event characterized by negative εHf values around -5.0 and representing recycling during Permian times of Mesoproterozoic crust (TDM of ∼1.5 Ga), and the Cañadón Asfalto magmatic event with negative (-8.2) to positive (+4) εHf values and Meso- to Neoproterozoic TDM between 1.5 and 0.8 Ga. The younger event is characterized by three main cycles: C1 related to the Lonco Trapial magmatism, C2 to the Las Chacritas volcanism, and C3 to the Puesto Almada volcanism. These cycles are related with Marifil, Chon Aike and El Quemado formations volcanics events of Patagonia and the Neuquén Basin during the Mesozoic.

Radiation-induced diamond crystallization: Origin of carbonados and its implications on meteorite nano-diamonds

USGS Publications Warehouse

Ozima, M.; Tatsumoto, M.

1997-01-01

Ten carbonados from Central Africa were studied for U-Th-Pb systematics. To extract U, Th, and Pb from the samples, we developed a cold combustion technique wherein diamond was burnt in liquid oxygen. The technique gave low blanks; 25-50 pg for Pb, 3 pg for U, and 5 pg for Th. After very thorough acid treatments of the carbonados with hot HNO3, HF, and HCl over one week, most of U, Th, and Pb were removed from the samples. Lead in the acid-leached diamonds was highly radiogenic (206Pb/204Pb up to 470). However, the amounts of U and Th in the acid-leached diamonds are too low to account for the radiogenic Pb even if we assume 4.5 Ga for the age of the diamonds. Therefore, we conclude that the radiogenic Pb was implanted into the diamonds from surroundings by means of recoil energy of radioactive decays of U and Th. From the radiogenic lead isotopic composition, we estimate a minimum age of 2.6 Ga and a maximum age of 3.8 Ga for the formation of the carbonados. The above findings of the implantation of recoiled radiogenic Pb into carbonados is consistent with the process of radiation-induced crystallization which was proposed for carbonado by Kaminsky (1987). We show from some theoretical considerations that when highly energetic particles, such as those emitted from radioactive decay of U and Th, interact with carbonaceous materials, they give rise to cascades of atomic disturbance (over regions of about a few nanometer), and the disturbed atoms are likely to recrystallize to form micro-diamonds because of increasing surface energy due to small size. The radiation-induced diamond formation mechanism may be relevant to the origin of nano-diamonds in primitive meteorites. Copyright ?? 1997 Elsevier Science Ltd.

Community-based Approaches to Improving Accuracy, Precision, and Reproducibility in U-Pb and U-Th Geochronology

NASA Astrophysics Data System (ADS)

McLean, N. M.; Condon, D. J.; Bowring, S. A.; Schoene, B.; Dutton, A.; Rubin, K. H.

2015-12-01

The last two decades have seen a grassroots effort by the international geochronology community to "calibrate Earth history through teamwork and cooperation," both as part of the EARTHTIME initiative and though several daughter projects with similar goals. Its mission originally challenged laboratories "to produce temporal constraints with uncertainties approaching 0.1% of the radioisotopic ages," but EARTHTIME has since exceeded its charge in many ways. Both the U-Pb and Ar-Ar chronometers first considered for high-precision timescale calibration now regularly produce dates at the sub-per mil level thanks to instrumentation, laboratory, and software advances. At the same time new isotope systems, including U-Th dating of carbonates, have developed comparable precision. But the larger, inter-related scientific challenges envisioned at EARTHTIME's inception remain - for instance, precisely calibrating the global geologic timescale, estimating rates of change around major climatic perturbations, and understanding evolutionary rates through time - and increasingly require that data from multiple geochronometers be combined. To solve these problems, the next two decades of uranium-daughter geochronology will require further advances in accuracy, precision, and reproducibility. The U-Th system has much in common with U-Pb, in that both parent and daughter isotopes are solids that can easily be weighed and dissolved in acid, and have well-characterized reference materials certified for isotopic composition and/or purity. For U-Pb, improving lab-to-lab reproducibility has entailed dissolving precisely weighed U and Pb metals of known purity and isotopic composition together to make gravimetric solutions, then using these to calibrate widely distributed tracers composed of artificial U and Pb isotopes. To mimic laboratory measurements, naturally occurring U and Pb isotopes were also mixed in proportions to mimic samples of three different ages, to be run as internal standards and as measures of inter-laboratory reproducibility. The U-Th community is undertaking many of the same protocols, and has recently created publicly available gravimetric solutions, and large volumes of three age solutions for widespread distribution and inter-laboratory comparison.

Ion microprobe U-Pb dating and strontium isotope analysis of biogenic apatite

NASA Astrophysics Data System (ADS)

Sano, Y.; Toyoshima, K.; Takahata, N.; Shirai, K.

2012-12-01

Conodonts are micro-fossils chemically composed of apatite which occurred in the body of one animal. They are guide fossils to show formation ages of sedimentary sequences with the highest resolution [1] and good samples to verify the dating method. We developed the ion microprobe U-Pb dating of apatite [2] and applied the method to a Carboniferous conodont [3] by using a SHRIMP II installed at Department of Earth and Planetary Sciences, Hiroshima University. Recently we have developed the NanoSIMS U-Pb dating method and successfully measured the formation ages of monazite [4] and zircon [5] at Atmosphere and Ocean Research Institute, University of Tokyo. In this work we carried out the NanoSIMS U-Pb dating of biogenic apatite such as conodont. Since the spot size of NanoSIMS is smaller than SHRIMP II, it is easier to have multi-spots on the single fragment of biogenic apatite. Based on the isochron method of U-Pb system, we have calculated the formation ages. They are consistent with those in literature. In order to study the chemical evolution of ocean during the past 600 Million years, strontium isotopes (87Sr/86Sr) of fossil marine carbonate such as coral skeletons and foraminifera tests were measured and compiled [6]. However they are not robust when the age is older than 500Ma, partly due to post-depositional histories. Apatite is more stable and more resistant to the alteration than carbonate [7]. Recently we have developed the method of NanoSIMS strontium isotopic analysis of a fish otolith, which composed of aragonite [8]. In this work we carried out the strontium isotopic analysis of biogenic apatite. The advantage of the ion microprobe technique over the TIMS (thermal ionization mass spectrometer) and MC-ICP-MS (multi-collector inductively coupled argon plasma mass spectrometer) method is preservation of the important textural context and to provide an opportunity for other simultaneous analytical work with high spatial resolution. This is the case for the combination of U-Pb dating and strontium isotope analysis of biogenic apatite. This method may be useful to extract the information of the chemistry of Past ocean in future. [1] Sweet and Donoghue (2001) J. Paleont. 75, 1174-1184. [3] Sano et al., (1999) Chem. Geol. 153, 249-258. [3] Sano and Terada (2001) Geophys. Res. Lett. 28, 831-834. [4] Sano et al. (2006) Geochem. J. 40, 597-608. [5] Takahata et al. (2008) Gondwana Res. 14, 587-596. [6] Prokoph et al. (2008) Earth Sci. Rev. 87, 113-133. [7] Karhu and Epstein (1986) Geochim. Cosmochim. Acta 50, 1745-1756. [8] Sano et al. (2008) App. Geochem. 23, 2406-2413.

Combined oxygen-isotope and U-Pb zoning studies of titanite: New criteria for age preservation

DOE PAGES

Bonamici, Chloe E.; Fanning, C. Mark; Kozdon, Reinhard; ...

2015-02-11

Here, titanite is an important U-Pb chronometer for dating geologic events, but its high-temperature applicability depends upon its retention of radiogenic lead (Pb). Experimental data predict similar rates of diffusion for lead (Pb) and oxygen (O) in titanite at granulite-facies metamorphic conditions (T = 650-800°C). This study therefore investigates the utility of O-isotope zoning as an indicator for U-Pb zoning in natural titanite samples from the Carthage-Colton Mylonite Zone of the Adirondack Mountains, New York. Based on previous field, textural, and microanalytical work, there are four generations (types) of titanite in the study area, at least two of which preservemore » diffusion-related δ 18O zoning. U-Th-Pb was analyzed by SIMS along traverses across three grains of type-2 titanite, which show well-developed diffusional δ 18O zoning, and one representative grain from each of the other titanite generations.« less

Prospects for Practical Laser Ablation U/Pb and (U-Th)/He Double-Dating (LADD) of Detrital Apatite

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2017-12-01

A laser ablation micro-analytical technique for (U-Th)/He dating has been shown to be an effective approach to the thermochronologic study of detrital zircons (Tripathy-Lang et al., J. Geophys. Res., 2013), while Evans et al. (J. Anal. At. Spectrom., 2015) and Horne et al. (Geochim. Cosmochim. Acta, 2016) demonstrated how the technique could be modified to enable laser ablation U/Pb and (U-Th)/He double-dating (LADD) of detrital zircon and titanite. These successes beg the question of whether or not LADD is viable for another commonly encountered detrital mineral: apatite. Exploratory LADD studies in Arizona State University's Group 18 Laboratories - using Durango fluorapatite, apatite from the Fish Canyon tuff, and detrital apatite from modern fluvial sediments in the eastern Sierra Nevada of California - illustrate that the method is indeed viable for detrital apatite. However, the method may not be appropriate for all detrital samples. For example, many apatite grains encountered in detrital samples from young orogenic settings have low concentrations of U and Th and small crystal sizes. This can lead to imprecise laser ablation (U-Th)/He dates, especially for very young grains potentially obscuring or inhibiting relevant interpretations of the data set.

The mesoproterozoic Beaverhead impact structure and its tectonic setting, Montana-Idaho: 40Ar/39 and U-Pb isotopic constraints

USGS Publications Warehouse

Kellogg, K.S.; Snee, L.W.; Unruh, D.M.

2003-01-01

New 40Ar/39Ar and uranium-lead (U-Pb) zircon data from the Beaverhead impact structure, first identified by extensive shatter coning of Proterozoic quartzite and gneiss from the Beaverhead Mountains near the Montana-Idaho border, indicate that the structure formed at or after 900 Ma. The 40Ar/39Ar age spectra from fine-grained muscovite and biotite from a breccia zone in high-grade gneiss show significant argon loss but yield dates for highest-temperature steps that cluster between 899 and 908 Ma. The dated minerals probably formed by recrystallization of impact glass, so on both geologic and isotopic grounds, the dates probably represent the minimum age of impact. U-Pb data for zircons from the same breccia are strongly discordant and yield an upper intercept apparent age of 2464 ?? 56 Ma and a lower intercept apparent age of 779 ?? 69 Ma. Another brecciated gneiss about 7 km to the northeast that does not contain secondary mica does contain zircons that yield a concordant apparent age of 2455 ?? 9 Ma. Nearby gneiss that neither is brecciated nor contains shatter cones yields an apparent age of 2451 ?? 46 Ma. The 40Ar/39Ar results constrain the age of the shatter-coned quartzite and indicate that it is >900 Ma and possibly correlative with the Gunsight Formation of the Mesoproterozoic Lemhi Group. The upper intercept U-Pb age of ???2450 Ma from all three dated samples also shows that the Paleoproterozoic basement rocks of the area are among the youngest in the mostly Archean Wyoming province of North America. The impact site lies near the margin of the province, along the northeast-trending Great Falls tectonic zone, and the relatively young crustal age may reflect Early Proterozoic marginal accretion.

In situ location and U-Pb dating of small zircon grains in igneous rocks using laser ablation-inductively coupled plasma-quadrupole mass spectrometry

NASA Astrophysics Data System (ADS)

Sack, Patrick J.; Berry, Ron F.; Meffre, Sebastien; Falloon, Trevor J.; Gemmell, J. Bruce; Friedman, Richard M.

2011-05-01

A new U-Pb zircon dating protocol for small (10-50 μm) zircons has been developed using an automated searching method to locate zircon grains in a polished rock mount. The scanning electron microscope-energy-dispersive X ray spectrum-based automated searching method can routinely find in situ zircon grains larger than 5 μm across. A selection of these grains was ablated using a 10 μm laser spot and analyzed in an inductively coupled plasma-quadrupole mass spectrometer (ICP-QMS). The technique has lower precision (˜6% uncertainty at 95% confidence on individual spot analyses) than typical laser ablation ICP-MS (˜2%), secondary ion mass spectrometry (<1%), and isotope dilution-thermal ionization mass spectrometry (˜0.4%) methods. However, it is accurate and has been used successfully on fine-grained lithologies, including mafic rocks from island arcs, ocean basins, and ophiolites, which have traditionally been considered devoid of dateable zircons. This technique is particularly well suited for medium- to fine-grained mafic volcanic rocks where zircon separation is challenging and can also be used to date rocks where only small amounts of sample are available (clasts, xenoliths, dredge rocks). The most significant problem with dating small in situ zircon grains is Pb loss. In our study, many of the small zircons analyzed have high U contents, and the isotopic compositions of these grains are consistent with Pb loss resulting from internal α radiation damage. This problem is not significant in very young rocks and can be minimized in older rocks by avoiding high-U zircon grains.

Progress integrating ID-TIMS U-Pb geochronology with accessory mineral geochemistry: towards better accuracy and higher precision time

NASA Astrophysics Data System (ADS)

Schoene, B.; Samperton, K. M.; Crowley, J. L.; Cottle, J. M.

2012-12-01

It is increasingly common that hand samples of plutonic and volcanic rocks contain zircon with dates that span between zero and >100 ka. This recognition comes from the increased application of U-series geochronology on young volcanic rocks and the increased precision to better than 0.1% on single zircons by the U-Pb ID-TIMS method. It has thus become more difficult to interpret such complicated datasets in terms of ashbed eruption or magma emplacement, which are critical constraints for geochronologic applications ranging from biotic evolution and the stratigraphic record to magmatic and metamorphic processes in orogenic belts. It is important, therefore, to develop methods that aid in interpreting which minerals, if any, date the targeted process. One promising tactic is to better integrate accessory mineral geochemistry with high-precision ID-TIMS U-Pb geochronology. These dual constraints can 1) identify cogenetic populations of minerals, and 2) record magmatic or metamorphic fluid evolution through time. Goal (1) has been widely sought with in situ geochronology and geochemical analysis but is limited by low-precision dates. Recent work has attempted to bridge this gap by retrieving the typically discarded elution from ion exchange chemistry that precedes ID-TIMS U-Pb geochronology and analyzing it by ICP-MS (U-Pb TIMS-TEA). The result integrates geochemistry and high-precision geochronology from the exact same volume of material. The limitation of this method is the relatively coarse spatial resolution compared to in situ techniques, and thus averages potentially complicated trace element profiles through single minerals or mineral fragments. In continued work, we test the effect of this on zircon by beginning with CL imaging to reveal internal zonation and growth histories. This is followed by in situ LA-ICPMS trace element transects of imaged grains to reveal internal geochemical zonation. The same grains are then removed from grain-mount, fragmented, and analyzed by U-Pb TIMS-TEA. In situ trace element transects are used to model predicted TIMS-TEA trace element concentrations to test whether complicated trace element profiles undermine U-Pb TIMS-TEA data. We find good agreement between predicted and measured TIMS-TEA data, and can argue that the measured ID-TIMS U-Pb date corresponds to the time at which the geochemical signature measured by TIMS-TEA was acquired. Thus, in a hypothetical magma that is differentiating through AFC processes on timescales resolvable by geochronology, U-Pb TIMS-TEA should usually be a robust indicator of magma evolution through time. We present data from two ca. 40-30 Ma alpine intrusions from northern Italy: the southern Adamello batholith and the Bergell intrusion. The relatively young age of these intrusions permits uncertainties on individual zircon or zircon fragments as good as 10 ka, while zircon populations from individual hand samples often record zircon growth of >200 ka. Using the methodologies described above, we explore whether these zircons record in situ magmatic differentiation or introduction of antecrystic zircon to magma batches, and integrate these data to gain a better understanding of magma storage, differentiation and emplacement as a function of pressure, temperature, and time. These methods are a promising step towards interpreting complicated geochronologic data in ashbed samples as well through a better understanding of magmatic processes that precede eruption.

SHRIMP U-Pb detrital zircon dating to check subdivisions in metamorphic complexes: a case of study in the Nevado-Filábride complex (Betic Cordillera, Spain)

NASA Astrophysics Data System (ADS)

Santamaría-López, Ángel; Sanz de Galdeano, Carlos

2018-04-01

U-Pb dating on inherited detrital zircons has been applied to obtain the probable maximum age of deposition of the detrital protolith of the Nevado-Filábride complex (Betic Cordillera, Spain). Five of eight samples correspond to the lower part of the lithologic sequence of this complex, where radiometric dating of metasediments has not been presented till the present. The youngest age populations in the majority of samples are Carboniferous. The estimation of the maximum age of deposition in the lower and upper units is 349.1 ± 1.6 and 334.6 ± 2.9 Ma, respectively. In addition, samples show common age populations at ca. 490-630 and ca. 910-1010 Ma. Observations agree with the Carboniferous to early Permian U-Pb ages previously obtained in orthogneisses levels which are situated in the upper part of the complex. Combination of the minimum age of deposition deducible from the orthogneisses studies and the maximum ages of deposition obtained from the detrital zircons of this work, allow establishing the deposition of de studied lithological succession comprised between ca. 282 and 349 Ma or a shorter period.

3.1 Ga crystallization age for magnesian and ferroan gabbro lithologies in the Northwest Africa 773 clan of lunar meteorites

NASA Astrophysics Data System (ADS)

Shaulis, B. J.; Righter, M.; Lapen, T. J.; Jolliff, B. L.; Irving, A. J.

2017-09-01

The Northwest Africa (NWA) 773 clan of meteorites is a group of paired and/or petrogenetically related stones that contain at least six different lithologies: magnesian gabbro, ferroan gabbro, anorthositic gabbro, olivine phyric basalt, regolith breccia, and polymict breccia. Uranium-lead dates of baddeleyite in the magnesian gabbro, ferroan gabbro, and components within breccia lithologies of paired lunar meteorites NWA 773, NWA 3170, NWA 6950, and NWA 7007 indicate a chronologic link among the meteorites and their components. A total of 50 baddeleyite grains were analyzed and yielded weighted average 207Pb-206Pb dates of 3119.4 ± 9.4 (n = 27), 3108 ± 20 (n = 13), and 3113 ± 15 (n = 10) Ma for the magnesian gabbro, ferroan gabbro, and polymict breccia lithologies, respectively. A weighted average date of 3115.6 ± 6.8 Ma (n = 47/50) was calculated from the baddeleyite dates for all lithologies. A single large zircon grain found in a lithic clast in the polymict breccia of NWA 773 yielded a U-Pb concordia date of 3953 ± 18 Ma, indicating a much more ancient source for some of the components within the breccia. A U-Pb concordia date of apatite and merrillite grains from the magnesian gabbro and polymict breccia lithologies in NWA 773 is 3112 ± 33 Ma, identical to the baddeleyite dates. Magnesian and ferroan gabbros, as well as the dated baddeleyite and Ca-phosphate-bearing detritus in the breccia lithologies, formed during the same igneous event at about 3115 Ma. These data also strengthen proposed petrogenetic connections between magnesian and ferroan gabbro lithologies, which represent some of the youngest igneous rocks known from the Moon.

Ion-probe U–Pb dating of authigenic and detrital opal from Neogene-Quaternary alluvium

USGS Publications Warehouse

Neymark, Leonid; Paces, James B.

2013-01-01

Knowing depositional ages of alluvial fans is essential for many tectonic, paleoclimatic, and geomorphic studies in arid environments. The use of U–Pb dating on secondary silica to establish the age of Neogene-Quaternary clastic sediments was tested on samples of authigenic and detrital opal and chalcedony from depths of ∼25 to 53 m in boreholes at Midway Valley, Nevada. Dating of authigenic opal present as rinds on rock clasts and in calcite/silica cements establishes minimum ages of alluvium deposition; dating of detrital opal or chalcedony derived from the source volcanic rocks gives the maximum age of sediment deposition.Materials analyzed included 12 samples of authigenic opal, one sample of fracture-coating opal from bedrock, one sample of detrital opal, and two samples of detrital chalcedony. Uranium–lead isotope data were obtained by both thermal ionization mass spectrometry and ion-microprobe. Uranium concentrations ranged from tens to hundreds of μg/g. Relatively large U/Pb allowed calculation of 206Pb/238U ages that ranged from 1.64±0.36 (2σ) to 6.16±0.50 Ma for authigenic opal and from 8.34±0.28 to 11.2±1.3 Ma for detrital opal/chalcedony. Three samples with the most radiogenic Pb isotope compositions also allowed calculation of 207Pb/235U ages, which were concordant with 206Pb/238U ages from the same samples.These results indicate that basin development at Midway Valley was initiated between about 8 and 6 Ma, and that the basin was filled at long-term average deposition rates of less than 1 cm/ka. Because alluvium in Midway Valley was derived from adjacent highlands at Yucca Mountain, the low rates of deposition determined in this study may imply a slow rate of erosion of Yucca Mountain. Volcanic strata underlying the basin are offset by a number of buried faults to a greater degree than the relatively smooth-sloping bedrock/alluvium contact. These geologic relations indicate that movement on most faults ceased prior to erosional planation and burial. Therefore, ages of the authigenic opal from basal alluvium indicate that the last movement on buried faults was older than about 6 Ma.

Apollo 12 breccia 12013: Impact-induced partial Pb loss in zircon and its implications for lunar geochronology

NASA Astrophysics Data System (ADS)

Thiessen, F.; Nemchin, A. A.; Snape, J. F.; Bellucci, J. J.; Whitehouse, M. J.

2018-06-01

Apollo 12 breccia 12013 is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. The zircon grains within the grey portion yielded a large range of ages (4154 ± 7 to 4308 ± 6 Ma, 2σ) and show decreasing U and Th concentrations within the younger grains. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts. The latter process requires high temperatures above 1600-1700 °C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. As a result of the high temperatures, the U-Pb system of the remaining original zircon grains was most likely open for Pb diffusion causing partial resetting and the observed range of 207Pb/206Pb ages. The event that led to the Pb loss in zircon could potentially be dated by the U-Pb system in Ca-phosphates, which have a weighted average 207Pb/206Pb age across both lithologies of 3924 ± 3 Ma (95% conf.). This age is identical within error to the combined average 207Pb/206Pb age of 3926 ± 2 Ma that was previously obtained from Ca-phosphates within Apollo 14 breccias, zircon grains in Apollo 12 impact melt breccias, and the lunar meteorite SaU 169. This age was interpreted to date the Imbrium impact. The zircon grains located within the black portion of the breccia yielded a similar range of ages (4123 ± 13 to 4328 ± 14 Ma, 2σ) to those in the grey portion. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry. Similarly, the currently available zircon data set for all lunar samples may be distorted by partial Pb loss, resulting in meaningless and misleading age distribution patterns. Therefore, it is crucial to fully understand and recognize the processes and conditions that may lead to partial resetting of the U-Pb system in zircon in order to better constrain the magmatic and impact history of the Moon.

Approaches for the accurate definition of geological time boundaries

NASA Astrophysics Data System (ADS)

Schaltegger, Urs; Baresel, Björn; Ovtcharova, Maria; Goudemand, Nicolas; Bucher, Hugo

2015-04-01

Which strategies lead to the most precise and accurate date of a given geological boundary? Geological units are usually defined by the occurrence of characteristic taxa and hence boundaries between these geological units correspond to dramatic faunal and/or floral turnovers and they are primarily defined using first or last occurrences of index species, or ideally by the separation interval between two consecutive, characteristic associations of fossil taxa. These boundaries need to be defined in a way that enables their worldwide recognition and correlation across different stratigraphic successions, using tools as different as bio-, magneto-, and chemo-stratigraphy, and astrochronology. Sedimentary sequences can be dated in numerical terms by applying high-precision chemical-abrasion, isotope-dilution, thermal-ionization mass spectrometry (CA-ID-TIMS) U-Pb age determination to zircon (ZrSiO4) in intercalated volcanic ashes. But, though volcanic activity is common in geological history, ashes are not necessarily close to the boundary we would like to date precisely and accurately. In addition, U-Pb zircon data sets may be very complex and difficult to interpret in terms of the age of ash deposition. To overcome these difficulties we use a multi-proxy approach we applied to the precise and accurate dating of the Permo-Triassic and Early-Middle Triassic boundaries in South China. a) Dense sampling of ashes across the critical time interval and a sufficiently large number of analysed zircons per ash sample can guarantee the recognition of all system complexities. Geochronological datasets from U-Pb dating of volcanic zircon may indeed combine effects of i) post-crystallization Pb loss from percolation of hydrothermal fluids (even using chemical abrasion), with ii) age dispersion from prolonged residence of earlier crystallized zircon in the magmatic system. As a result, U-Pb dates of individual zircons are both apparently younger and older than the depositional age of the ash, therefore masking the true age of deposition. Trace element ratios such as Th/U, Yb/Gd, as well as Hf isotope analysis of dated zircon can be used to decipher the temporal evolution of the magmatic system before the eruption and deposition of the studied ashes, and resolve the complex system behaviour of the zircons. b) Changes in the source of the magma may happen between the deposition of two stratigraphically consecutive ash beds. They result in the modification of the trace element signature of zircon, but also of apatite (Ca5 (F, Cl, OH) (PO4)3). Trace element characteristics in apatite (e.g. Mg, Mn, Fe, F, Cl, Ce, and Y) are a reliable tool for distinguishing chemically similar groups of apatite crystals to unravel the geochemical fingerprint of one single ash bed. By establishing this fingerprint, ash beds of geographically separated geologic sections can be correlated even if they have not all been dated by U-Pb techniques. c) The ultimate goal of quantitative stratigraphy is to establish an age model that predicts the age of a synchronous time line with an associated 95% confidence interval for any such line within a stratigraphic sequence. We show how a Bayesian, non-parametric interpolation approach can be applied to very complex data sets and leads to a well-defined age solution, possibly identifying changes in sedimentation rate. The age of a geological time boundary bracketed by dated samples in such an age model can be defined with an associated uncertainty.

Implications of Bishop Tuff zircon U-Pb ages for rates of zircon growth and magma accumulation

NASA Astrophysics Data System (ADS)

Reid, M. R.; Schmitt, A. K.

2012-12-01

Rates of geologic processes obtained from natural studies rely on accurate geochronologic information. An important benchmark in geochronology as well as a valuable source of insights into the evolution of voluminous explosive eruptions is the >600 km3 Bishop Tuff (BT). A recently determined weighted mean 206Pb/238U date of 767.1±0.9 ka for a BT zircon population [1] is indistinguishable from the recalibrated 40Ar/39Ar sanidine date of 767.4±2.2 ka [2], potentially providing a key intercalibration point between astronomical and radio-isotopic dating approaches. Consequences of these results are linear zircon growth rates of >1×10-14 cm/sec and magma accumulation rates of >200 km3/ka. In contrast, spatially selective SIMS U-Pb dating of BT zircons yielded mean pre-eruption ages of 850 ka [3], a difference that raises questions about the validity of intercalibration between U-Pb and K-Ar dating methods and the history of magma accumulation. We obtained new SIMS analyses of the BT zircons using more spatially and analytically sensitive methods and verifying our accuracy against the TIMS dated Quaternary zircon 61.308A (2.488±0.002 Ma). Analyses were performed on zircon rims and on oriented cross-sections exposed during optical interferometry-calibrated serial sectioning removing the outermost ~31 μm. Sputtering by a 100 nA ion beam versus the normally employed 10-12 nA beam resulted in enhanced radiogenic Pb yields and analytical uncertainties for Quaternary zircon approaching the U-Pb age reproducibility of the primary zircon standard (~1-2 % for AS3). Ages obtained at ~31 μm depth (representing <5% of crystal growth in most cases) average 892±26ka (MSWD=0.29), corroborating previous evidence for residence times of several tens of ka. Rim ages average 781±22 ka (MSWD=0.61), overlapping Ar/Ar determinations of eruption age and corroborating the importance of near-eruption aged zircon growth. Our results confirm the presence of BT zircon domains that predate eruption by tens of ka, domains that could be difficult to detect by single crystal analyses. SIMS analyses can selectively sample more interior portions of grains whereas whole grain TIMS analyses will be volumetrically biased towards the outermost ~20% of the BT zircons. Spots centered on BT zircon cores sample a domain representing only a few volume percent of the crystal; if mantled completely by eruption-aged growth, single crystal zircon ages are predicted to be <5 ka older than eruption, extrapolating from the SIMS dates. When, compared to grain interiors, the greater average near-rim U concentrations are factored into this mass balance, the influence of these older domains is further diminished. A scenario that can at least partially reconcile the disparate zircon dates in the context of evolution of the BT and the dynamics of zircon crystallization is as follows: (1) zircon nucleation and relatively rapid growth an average of ~100 ka before eruption; (2) significant decay in zircon growth rates as temperatures and Zr melt gradients relax; and, finally, (3) mantling of earlier zircon cores by rejuvenation and renewed BT zircon growth during the lead-up to eruption. Comparisons between spot and single grain dating show that most zircon growth likely occurred close to eruption. [1] Crowley et al., 2007.Geology 35, 1123. [2] Rivera et al., 2011. Earth Planet. Sci. Lett. 311, 420. [3] Simon and Reid, 2005. Earth Planet. Sci. Lett. 235, 123.

Permian arc-back-arc basin development along the Ailaoshan tectonic zone: Geochemical, isotopic and geochronological evidence from the Mojiang volcanic rocks, Southwest China

NASA Astrophysics Data System (ADS)

Fan, Weiming; Wang, Yuejun; Zhang, Aimei; Zhang, Feifei; Zhang, Yuzhi

2010-10-01

This paper presents a set of new SHRIMP zircon U-Pb geochronological, elemental and Sr-Nd-Pb isotopic data for the Wusu and Yaxuanqiao basaltic rocks (the Mojiang area) along the Ailaoshan tectonic zone. The Wusu basaltic sequence is dominated by SiO 2-poor, MgO- and TiO 2-rich basalts with a major mineral assemblage of plagioclase + clinopyroxene. These rocks gave a SHRIMP zircon U-Pb age of 287 ± 5 Ma (MSWD = 0.58). In contrast, the Yaxuanqiao basaltic sequence is predominantly composed of high-Al basaltic andesite, which gave a SHRIMP zircon U-Pb age of 265 ± 7 Ma (MSWD = 0.34). The analyzed samples for both sequences exhibit significant enrichment in LILEs and depletion in HFSEs with (Nb/La)n of 0.38-0.81, similar to arc-like volcanics. They have positive ɛNd(t) values (+ 3.52 to + 5.54). In comparison with MORB-derived magmatic rocks, the Wusu basalts are more enriched in LILEs and REEs, and the Yaxuanqiao samples are more enriched in LILEs but variably depleted in Ti, Y and HREE. The Wusu samples show high Pb isotopic ratios, similar to the Tethyan basalts, whereas the Yaxuanqiao samples plot in the field of the global pelagic sediments. The geochemical and Sr-Nd-Pb isotopic characteristics suggest that the Wusu basalts originated from a MORB-like source metasomatised by slab-derived fluids, while the Yaxuanqiao rocks have a fluid-modified MORB source with the input of subducted sediments. The geochemical affinity to both MORB- and arc-like sources, together with other geological observations, appears to support the development of a Permian arc-back-arc basin along the Ailaoshan-Song Ma tectonic zone in response to the northward subduction of the Paleotethys main Ocean. The final closure of the arc-back-arc basin took place in the uppermost Triassic due to the diachronous amalgamation between the Yangtze and Simao-Indochina Blocks.

A comparison of lead pollution record in Sphagnum peat with known historical Pb emission rates in the British Isles and the Czech Republic

NASA Astrophysics Data System (ADS)

Novak, Martin; Erel, Yigal; Zemanova, Leona; Bottrell, Simon H.; Adamova, Marie

Vertical Pb concentration gradients and isotope ratios ( 206Pb/ 207Pb, 208Pb/ 207Pb) are reported for five 210Pb-dated Sphagnum peat profiles. The studied peat bogs are in the British Isles (Thorne Moors, England; Mull, Scotland; and Connemara, Eire) and central Europe (Ocean, northern Czech Republic; Rybarenska slat, southern Czech Republic). Both the U.K. and the Czech Republic experienced maximum Pb emissions from Ag-Pb smelting around 1880. Pb emissions from coal burning peaked in 1955 in the U.K. and in the 1980s in the Czech Republic. In both countries, use of alkyl-lead additives to gasoline resulted in large Pb emissions between 1950 and 2000. We hypothesized that peaks in Pb emissions from smelting, coal burning and gasoline burning, respectively, should be mirrored in the peat profiles. However, a more complicated pattern emerged. Maximum annual Pb accumulation rates occurred in 1870 at Ocean, 1940 at Thorne Moors, 1988 at Rybarenska slat, and 1990 at Mull and Connemara. Atmospheric Pb inputs decreased in the order Thorne Moors ≥ Ocean > Rybarenska slat > Mull > Connemara. The Ocean bog was unique in the central European region in that its maximum Pb pollution dated back to the 19th century and coincided with maximum Pb smelting at Freiberg and Pribram. In contrast, numerous previously studied sites showed no Pb accumulation maximum in the 19th century, but increasing pollution until the 1980s. It remains unclear why Ocean did not record the regional peak in Pb emissions caused by high coal and gasoline burning around 1980, while an array of nearby bogs studied previously did record the 1980 coal/gasoline peak, but no 1880 smelting peak. Mean 206Pb/ 207Pb ratios of potential pollution sources were 1.07 and 1.11 for gasoline, 1.17 and 1.17 for local ores, and 1.18 and 1.19 for coal in the U.K. and the Czech Republic, respectively. The calculated percentages of gasoline-derived Pb in peat (≤55% for the British Isles and ≤63% for the Czech Republic) were surprisingly low. An explanation for the low percentage of gasoline-derived Pb in peat can be more easily found for the Czech sites (until 1989 Czechoslovakia was the third largest lignite producer in the world). Regional differences in deposition rates of gasoline-derived Pb in the U.K. need further study.

CHIME monazite dating using FE-EPMA equipped with R=100 mm spectrometers

NASA Astrophysics Data System (ADS)

Shibata, K.; Shimizu, M.; Suzuki, K.; Sueoka, S.; Niwa, M.

2015-12-01

The age spectrum of detrital monazite grains is used to unravel the tectono-thermal history of the pre-Neogene terranes, which is required for geological disposal of high-level radioactive waste on tectonically active Japanese Islands. The CHIME (Chemical Th-U-total Pb isochron method, Suzuki and Adachi, 1991) is best suited for dating of detrital monazite whose grains are not chronologically uniform. In the previous studies (eg, Suzuki, 2011), EPMA equipped with conventional R=140 mm spectrometers was used for measurement of U, Th and Pb. However the spectrometers have low count rate of measurement of Pb. The JEOL JXA-8530F FE-EPMA equipped with R=100 mm spectrometers has been applied for the CHIME monazite dating. The intrinsic responses of each of the R=100 mm spectrometers for PbMβ are around ten times higher than that of the R=140 mm spectrometer. The R=100 mm spectrometers permits obtaining high count rate, which enables us to shorten measurement time than before. As these spectrometers have peculiar spectral interference, the method reported by Amli and Griffin (1978) is applied for correction of the interference. In order to verify the dating using the FE-EPMA and the interference correction method, two distinct age groups of monazite were measured. The ages were 425±25 Ma for monazite from Cooma granite in southeastern Australia, which had dated by SHRIMP as 432.8 ± 3.5 Ma (Williams, 2001) and 67 ± 7 Ma for monazite of the Kojaku granite in southwestern Japan, which is corresponding to the LA-ICP-MS U-Pb zircon ages of 68.5 ± 0.7 Ma. These results indicate that the FE-EPMA and the interference correction method are useful for the CHIME monazite dating and for revealing the tectono-thermal history of the terranes. This study was carried out under a contract with Agency of Natural Resources and Energy(ANRE), part of Ministry of Economy, Trade and Industry (METI) of Japan as part of its R&D supporting program for developing geological disposal technology.

Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar

USGS Publications Warehouse

Thomas, Ronald J.; De Waele, B.; Schofield, D.I.; Goodenough, K.M.; Horstwood, M.; Tucker, R.; Bauer, W.; Annells, R.; Howard, K. J.; Walsh, G.; Rabarimanana, M.; Rafahatelo, J.-M.; Ralison, A.V.; Randriamananjara, T.

2009-01-01

The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1:100 000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of 14 U-Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern "terranes", each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of "cratonic" Madagascar, during the main collisional phase of the East African Orogeny at ca. 540 Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U-Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477 Ma) and two volcano-sedimentary sequences (high-grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720-740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U-Pb SHRIMP ages between 705 and 718 Ma. Whole-rock geochemical data confirm the calc-alkaline, arc-related nature of the plutonic rocks. The volcanic rocks of the Daraina and Milanoa groups also show characteristics of arc-related magmatism, but include both calc-alkaline and tholeiitic compositions. It is not certain when the two Bemarivo terranes were juxtaposed, but ages from metamorphic rims on zircon suggest that both the northern and southern terranes were accreted to the northern cratonic margin of Madagascar at about 540-530 Ma. Terrane accretion included the assembly of the Archaean Antongil and Antananarivo cratons and the high-grade Neoproterozoic Anaboriana Belt. Late- to post-tectonic granitoids of the Maevarano Suite, the youngest plutons of which gave ca. 520 Ma ages, intrude all terranes in northern Madagascar showing that terrane accretion was completed by this time. ?? 2009 Natural Environment Research Council (NERC).

Thermal and exhumation history of the central Tianshan (NW China): Constraints by U-Pb geochronology and Ar-Ar and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Yin, J.; Chen, W.; Hodges, K. V.; Xiao, W.; Van Soest, M. C.; Cai, K.; Zhang, B.; Mercer, C. M.; Yuan, C.

2015-12-01

Geochronology and thermochronology using multiple mineral-isotopic chronometers reveals the thermo-tectonic history of the central Tianshan (NW China) from emplacement to exhumation. Granites from the central Tianshan, which are associated with the southward subduction of the northern Tianshan Ocean, have been dated at 362-354 Ma using the LA-ICP-MS Zircon U-Pb method. A younger diorite sample (282 ± 1 Ma, Zircon U-Pb method by LA-ICP-MS) from northern Tianshan formed during the final closure of the Northern Tianshan Ocean when the Junggar Block collided with the Yili-Central Tianshan Block. 40Ar/39Ar step-wise heating plateau dates (biotite Ar/Ar: 312-293 Ma; Plagioclase Ar/Ar: 270-229 Ma) from the Central Tianshan show rapid post-magmatic cooling during the Late Carboniferous-Early Permian followed by a more modest rate of cooling from the middle Permian to the middle Jurassic. The northern Tianshan diorite (biotite Ar/Ar: 240 ± 1 Ma) also reveals a middle Jurassic cooling. Apatite (U-Th )/He dates from the central Tianshan samples range from ca. 130 Ma to ca. 116 Ma. The Apatite (U-Th )/He date for the northern Tianshan sample is ca. 27 Ma. Previous studies also reported Apatite (U-Th)/He ages of ca. 44 Ma-11 Ma in the Baluntai area of the southern Central Tianshan[1]. Two episodes of cooling are distinguished by thermal history modelling: (1) Mesozoic cooling occurred as the result of the exhumation and tectonic reactivation of the central Tianshan; and (2) The Tianshan orogenic belt has been rapidly exhumed since the Middle Cenozoic. References [1] Lü, H.H., Chang, Y., Wang, W., Zhou, Z.Y., 2013. Rapid exhumation of the Tianshan Mountains since the early Miocene: Evidence from combined apatite fission track and (U-Th)/He thermochronology. Science China: Earth Sciences, 43(12): 1964-1974 (in Chinese).

The double-edged sword of high-precision U-Pb geochronology or be careful what you wish for. (Invited)

NASA Astrophysics Data System (ADS)

Bowring, S. A.

2010-12-01

Over the past two decades, U-Pb geochronology by ID-TIMS has been refined to achieve internal (analytical) uncertainties on a single grain analysis of ± ~ 0.1-0.2%, and 0.05% or better on weighted mean dates. This level of precision enables unprecedented evaluation of the rates and durations of geological processes, from magma chamber evolution to mass extinctions and recoveries. The increased precision, however, exposes complexity in magmatic/volcanic systems and highlights the importance of corrections related to disequilibrium partitioning of intermediate daughter products, and raises questions as to how best to interpret the complex spectrum of dates characteristic of many volcanic rocks. In addition, the increased precision requires renewed emphasis on the accuracy of U decay constants, the isotopic composition of U, the calibration of isotopic tracers, and the accurate propagation of uncertainties It is now commonplace in the high precision dating of volcanic ash-beds to analyze 5-20 single grains of zircon in an attempt to resolve the eruption/depositional age. Data sets with dispersion far in excess of analytical uncertainties are interpreted to reflect Pb-loss, inheritance, and protracted crystallization, often supported with zircon chemistry. In most cases, a weighted mean of the youngest reproducible dates is interpreted as the time of eruption/deposition. Crystallization histories of silicic magmatic systems recovered from plutonic rocks may also be protracted, though may not be directly applicable to silicic eruptions; each sample must be evaluated independently. A key to robust interpretations is the integration high-spatial resolution zircon trace element geochemistry with high-precision ID-TIMS analyses. The EARTHTIME initiative has focused on many of these issues, and the larger subject of constructing a timeline for earth history using both U-Pb and Ar-Ar chronometers. Despite continuing improvements in both, comparing dates for the same rock with both chronometers is not straightforward. Compelling issues range from pre-eruptive magma chamber residence, recognizing open system behavior, accurately correcting for disequilibrium amounts of 230Th and 231Pa, precise and accurate dates of fluence monitors for 40Ar/39Ar, and inter-laboratory biases. At present, despite the level of internal precision achievable by each technique, obstacles remain to combining both chronometers.

A combined Sm-Nd, Rb-Sr, and U-Pb isotopic study of Mg-suite norite 78238: Further evidence for early differentiation of the Moon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Edmunson, J; E.Borg, L; Nyquist, L E

2008-11-17

Lunar Mg-suite norite 78238 was dated using the Sm-Nd, Rb-Sr, and U-Pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of Mg-suite magmatism, as well as to provide a direct comparison between the three isotopic systems. The Sm-Nd isotopic system yields a crystallization age for 78238 of 4334 {+-} 37 Ma and an initial {var_epsilon}{sub Nd}{sup 143} value of -0.27 {+-} 0.74. The age-initial {var_epsilon}{sub Nd}{sup 143} (T-I) systematics of a variety of KREEP-rich samples, including 78238 and other Mg-suite rocks, KREEP basalts, and olivine cumulate NWA 773, suggest that lunar differentiationmore » was completed by 4492 {+-} 61 Ma assuming a Chondritic Uniform Reservoir bulk composition for the Moon. The Rb-Sr isotopic systematics of 78238 were disturbed by post-crystallization processes. Nevertheless, selected data points yield two Rb-Sr isochrons. One is concordant with the Sm-Nd crystallization age, 4366 {+-} 53 Ma. The other is 4003 {+-} 95 Ma and is concordant with an Ar-Ar age for 78236. The {sup 207}Pb-{sup 206}Pb age of 4333 {+-} 59 Ma is concordant with the Sm-Nd age. The U-Pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the Pb-Pb system, and may reflect fractionation of U and Pb during sample handling. Despite the disturbed nature of the U-Pb systems, a time-averaged {mu} ({sup 238}U/{sup 204}Pb) value of the source can be estimated at 27 {+-} 30 from the Pb-Pb isotopic systematics. Because KREEP-rich samples are likely to be derived from source regions with the highest U/Pb ratios, the relatively low {mu} value calculated for the 78238 source suggests the bulk Moon does not have an exceedingly high {mu} value.« less

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

NASA Astrophysics Data System (ADS)

Kunz, Barbara E.; Regis, Daniele; Engi, Martin

2018-03-01

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.

Re-appraisal of the stratigraphy and determination of new U-Pb dates for the Sterkfontein hominin site, South Africa.

PubMed

Pickering, Robyn; Kramers, Jan D

2010-07-01

Sterkfontein Caves is the single richest early hominin site in the world with deposits yielding one or more species of Australopithecus and possible early Homo, as well as an extensive faunal collection. The inability to date the southern African cave sites accurately or precisely has hindered attempts to integrate the hominin fossil evidence into pan-African scenarios about human evolutionary history, and especially hominin biogeography. We have used U-Pb and U-Th techniques to date sheets of calcium carbonate flowstone inter-bedded between the fossiliferous sediments. For the first time, absolute age ranges can be assigned to the fossil-bearing deposits: Member 2 is between 2.8 +/- 0.28 and 2.6 +/- 0.30 Ma and Member 4 between 2.65 +/- 0.30 and 2.01 +/- 0.05 Ma. The age of 2.01 +/- 0.05 Ma for the top of Member 4 constrains the last appearance of Australopithecus africanus to 2 Ma. In the Silberberg Grotto we have reproduced the U-Pb age of approximately 2.2 Ma of for the flowstones associated with StW573. We believe that these deposits, including the fossil and the flowstones, accumulated rapidly around 2.2 Ma. The stratigraphy of the site is complex as sediments are exposed both in the underground chambers and at surface. We present a new interpretation of the stratigraphy based on surface mapping, boreholes logs and U-Pb ages. Every effort was made to retain the Member system, however, only Members 2 and 4 are recognized in the boreholes. We propose that the deposits formally known as Member 3 are in fact the distal equivalents of Member 4. The sediments of Members 2 and 4 consisted of cone-like deposits and probably never filled up the cave. The U-Th ages show that there are substantial deposits younger than 400 ka in the underground cave, underlying the older deposits, highlighting again that these cave fills are not simple layer-cakes.

Timing of global regression and microbial bloom linked with the Permian-Triassic boundary mass extinction: implications for driving mechanisms

NASA Astrophysics Data System (ADS)

Baresel, Bjoern; Bucher, Hugo; Bagherpour, Borhan; Brosse, Morgane; Guodun, Kuang; Schaltegger, Urs

2017-04-01

High-precision U-Pb dating of single-zircon crystals by chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) is applied to volcanic beds that are intercalated in sedimentary sequences across the Permian-Triassic boundary (PTB). By assuming that the zircon crystallization age closely approximate that of the volcanic eruption and subsequent deposition, U-Pb zircon geochronology is the preferred approach for dating abiotic and biotic events, such as the formational PTB and the Permian-Triassic boundary mass extinction (PTBME). We will present new U-Pb zircon dates for a series of volcanic ash beds in shallow-marine Permian-Triassic sections in the Nanpanjiang Basin, South China. These high-resolution U-Pb dates indicate a duration of 90 ± 38 kyr for the Permian sedimentary hiatus and a duration of 13 ± 57 kyr for the overlying Triassic microbial limestone in the shallow water settings of the Nanpanjiang pull apart Basin. The age and duration of the hiatus coincides with the formational PTB and the extinction interval in the Meishan Global Stratotype Section and Point, thus strongly supporting a glacio-eustatic regression, which best explains the genesis of the worldwide hiatus straddling the PTB in shallow water records. In adjacent deep marine troughs, rates of sediment accumulation display a six-fold decrease across the PTB compatible with a dryer and cooler climate during the Griesbachian as indicated by terrestrial plants. Our model of the PTBME hinges on the synchronicity of the hiatus with the onset of the Siberian Traps volcanism. This early eruptive phase likely released sulfur-rich volatiles into the stratosphere, thus simultaneously eliciting a short-lived ice age responsible for the global regression and a brief but intense acidification. Abrupt cooling, shrunk habitats on shelves and acidification may all have synergistically triggered the PTBME. Subsequently, the build-up of volcanic CO2 induced this transient cool climate whose early phase saw the deposition of the microbial limestone.

A Triassic to Cretaceous Sundaland-Pacific subduction margin in West Sarawak, Borneo

NASA Astrophysics Data System (ADS)

Breitfeld, H. Tim; Hall, Robert; Galin, Thomson; Forster, Margaret A.; BouDagher-Fadel, Marcelle K.

2017-01-01

Metamorphic rocks in West Sarawak are poorly exposed and studied. They were previously assumed to be pre-Carboniferous basement but had never been dated. New 40Ar/39Ar ages from white mica in quartz-mica schists reveal metamorphism between c. 216 to 220 Ma. The metamorphic rocks are associated with Triassic acid and basic igneous rocks, which indicate widespread magmatism. New U-Pb dating of zircons from the Jagoi Granodiorite indicates Triassic magmatism at c. 208 Ma and c. 240 Ma. U-Pb dating of zircons from volcaniclastic sediments of the Sadong and Kuching Formations confirms contemporaneous volcanism. The magmatic activity is interpreted to represent a Triassic subduction margin in westernmost West Sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the Sundaland-Pacific margin. West Sarawak and NW Kalimantan are underlain by continental crust that was already part of Sundaland or accreted to Sundaland in the Triassic. One metabasite sample, also previously assumed to be pre-Carboniferous basement, yielded Early Cretaceous 40Ar/39Ar ages. They are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism. U-Pb ages from detrital zircons in the Cretaceous Pedawan Formation are similar to those from the Schwaner granites of NW Kalimantan, and the Pedawan Formation is interpreted as part of a Cretaceous forearc basin containing material eroded from a magmatic arc that extended from Vietnam to west Borneo. The youngest U-Pb ages from zircons in a tuff layer from the uppermost part of the Pedawan Formation indicate that volcanic activity continued until c. 86 to 88 Ma when subduction terminated.

Status Report on the 40Ar/39Ar and U/Pb Dating of Tuffs in the Dewey Lake Formation of West Texas Towards Constraining the Permo-Triassic Magnetostratigraphic Time Scale

NASA Astrophysics Data System (ADS)

Chang, S.; Renne, P. R.; Mundil, R.

2007-12-01

A detailed magnetic polarity time scale for the Permo-Triassic Boundary interval, critical for correlating events in marine and terrestrial paleoenvironments, is not yet well-established. Recently, late Permian magnetostratigraphic studies have been reported for non-marine sections in Europe and South Africa (Szurlies et al., 2003; Nawrocki, 2004; Ward et al., 2005). However, these sections are devoid of index fossil suitable for correlation with marine successions and also lack age constraints from radioisotopic dating methods. In other words, it is dubious to correlate these magnetostratigraphic data with the GSSP Permo-Triassic boundary and mass extinction. The Dewey Lake red beds formation of West Texas, believed to be the youngest Permian formation in North America, has yielded high-quality paleomagnetic data (Molina-Garza et al., 1989; Steiner, 2001) and contains several silicic tuffs potentially enabling high-resolution calibration of the magnetic polarity time scale in this critical age range. The tuffs have yet to be placed into a regional stratigraphic or magnetostratigraphic framework, and it is unclear exactly how many distinct eruptive units are represented by the 7 distinct samples collected to date from widely separated (>160 km) localities. 40Ar/39Ar (sanidine and biotite) and U/Pb (zircon) studies reveal that all 7 sampled tuffs were probably erupted within several hundred ka of the Permo-Triassic boundary as dated at the Meishan GSSP section (Renne et al., 1995; Mundil et al., 2004) but results thus far are inadequate to convincingly resolve age differences between the various samples. U/Pb dating of some samples is severely challenged by Pb-loss from the zircons despite application of the Mattinson (2005) annealing/chemical abrasion technique. 40Ar/39Ar data have been obtained from as many as four different irradiations in order to reduce neutron fluence related error. We observe the familiar ~1% bias between U/Pb and 40Ar/39Ar ages. Biotite microprobe data, zircon U/Th TIMS data, and the absence of sanidine from some samples serve to help correlate or distinguish some samples despite irresolvable age differences; existing data suggest that 4 distinct tuffs are present in the Dewey Lake Formation. Resolving their ages convincingly will require further work, but it is clear from our results combined with previous magnetostratigraphic data that magnetic polarity reversals were relatively frequent in the latest Permian. Thus the uniqueness of correlations elsewhere with the Permo-Triassic boundary based on magnetostratigraphy alone are not well-founded.

The Mesozoic metamorphic-magmatic events in the Medog area, the Eastern Himalayan Syntaxis: constraints from zircon U-Pb geochronology, trace elements and Hf isotope compositions in granitoids

NASA Astrophysics Data System (ADS)

Dong, Hanwen; Xu, Zhiqin; Li, Yuan; Liu, Zhao; Li, Huaqi

2015-01-01

Based on the regional geological mapping, several granitoid intrusions had been discovered in the Eastern Himalayan Syntaxis (EHS). In order to constrain their petrogenesis and discuss their relations with the regional tectonics, we carried out U-Pb dating, trace elements and Hf isotope geochemistry studies on zircons separated from the granitoid rocks, in the area of the EHS. In this contribution, the granitoid rocks are mainly composed of diorites (X20-1-6) and granitic gneissic rocks (X2-15-1). The U-Pb zircon dating of diorites yields a crystallization age of 193.8 ± 2.0 Ma. These zircon have ɛ Hf( t) values ranging from -6.48 to -0.05, indicating an involvement of ancient crustal materials in the generation of these igneous rocks. The zircons from the Medog granitic gneissic rock commonly show zoning structures. The REE patterns and abundances of the inherited cores are different from those of the oscillatory rims. The LA-ICP-MS U-Pb zircon in situ analyses indicate that: (1) the zircon cores give multi-stage magmatic event ages ranging from 516 to 1,826 Ma, of which six ages are converged on 1,330-911 Ma, it is considered that the migmatitic gneiss has been formed in this time, and (2) while the zircon rims yield 206Pb/238U weighted mean ages of 217.4 ± 3.0 Ma (MSWD = 3.2), which was interpreted to represent the ages of the Triassic anatexis. Their ɛ Hf( t) values range from -18.98 to -8.36 and -14.22 to 8.72, respectively. The timing of the anatexis in the Medog area is coeval with the widespread metamorphism in Lhasa terrane.

Timescales of Crustal Cooling of the Superior Craton near Attawapiskat, Ontario, Canada, and Implications for Extent of Keweenawan Plume Heating.

NASA Astrophysics Data System (ADS)

Edwards, G. H.; Blackburn, T.; Smit, K.

2017-12-01

The thermal history of the Superior Craton was punctuated by a period of mantle plume heating at 1.1 Ga associated with the Keweenawan Rift, though the plume's spatial extent, temperature, and duration of heating remain unresolved. Kimberlites of Mesoproterozoic and Jurassic age in the Attawapiskat area, Northern Ontario contain lithospheric mantle and lower crustal xenoliths that record the thermal history 600km to the north of exposed 1.1 Ga Keweenawan volcanics and the topographically and gravimetrically defined plume center. Previous work on Attawapiskat kimberlites identified two populations of diamonds with differing thermal histories, suggesting two distinct phases of diamond growth. Corresponding geothermobarometric data indicate geotherm relaxation and broadening of the diamond stability field between the Mesoproterozoic and Jurassic. These data, however, do not uniquely resolve whether the region experienced significant heating coincident with Keweenawan rifting ( 1.1 Ga) or prolonged, unperturbed cooling since amalgamation of the Superior Craton ( 2.6 Ga). To discern between these two possible histories, we use accessory phase U-Pb thermochronology to construct a continuous thermal record of the lower crust. Here we present a dataset of U-Pb ID-TIMS measurements of rutile and apatite from xenoliths (n=8) sourced from the Jurassic age Victor Kimberlite. The U/Pb and Pb isotopic compositions of rutile and apatite from shallow-residing amphibolite xenoliths exhibit Proterozoic dates with a high degree of U-Pb discordance, reflecting slow cooling of the middle crust prior to 1.1 Ga. Granulite and eclogite xenoliths record younger dates consistent with their deeper sample residence, but with a high degree of U-Pb concordance that is inconsistent with continuous cooling through the Proterozoic. Reproducing the measured trend with numerical models requires a reheating event at 1.1 Ga. Imposing a 60-70mW/m2 geotherm at 1.1 Ga is high enough to replicate the observed U-Pb data but low enough to permit cooling that satisfies diamond thermal data. This indicates that the Keweenawan plume head extended to at least the Attawapiskat area, where plume heating abutted, and likely extended beyond, the associated topographic and gravity anomalies of the Superior Region.

Preface

NASA Astrophysics Data System (ADS)

Hart, Stan; Basu, Asish

Publication of this monograph will coincide, to a precision of a few per mil, with the centenary of Henri Becquerel's discovery of "radiations actives" (C. R. Acad. Sci., Feb. 24, 1896). In 1896 the Earth was only 40 million years old according to Lord Kelvin. Eleven years later, Boltwood had pushed the Earth's age past 2000 million years, based on the first U/Pb chemical dating results. In exciting progression came discovery of isotopes by J. J. Thomson in 1912, invention of the mass spectrometer by Dempster (1918) and Aston (1919), the first measurement of the isotopic composition of Pb (Aston, 1927) and the final approach, using Pb-Pb isotopic dating, to the correct age of the Earth: close—2.9 Ga (Gerling, 1942), closer—3.0 Ga (Holmes, 1949) and closest—4.50 Ga (Patterson, Tilton and Inghram, 1953).

Contemporaneous alkaline and tholeiitic magmatism in the Ponta Grossa Arch, Paraná-Etendeka Magmatic Province: Constraints from U-Pb zircon/baddeleyite and 40Ar/39Ar phlogopite dating of the José Fernandes Gabbro and mafic dykes

NASA Astrophysics Data System (ADS)

Almeida, Vidyã V.; Janasi, Valdecir A.; Heaman, Larry M.; Shaulis, Barry J.; Hollanda, Maria Helena B. M.; Renne, Paul R.

2018-04-01

We report the first high-precision ID-TIMS U-Pb baddeleyite/zircon and 40Ar/39Ar step-heating phlogopite age data for diabase and lamprophyre dykes and a mafic intrusion (José Fernandes Gabbro) located within the Ponta Grossa Arch, Brazil, in order to constrain the temporal evolution between Early Cretaceous tholeiitic and alkaline magmatism of the Paraná-Etendeka Magmatic Province. U-Pb dates from chemically abraded zircon data yielded the best estimate for the emplacement ages of a high Ti-P-Sr basaltic dyke (133.9 ± 0.2 Ma), a dyke with basaltic andesite composition (133.4 ± 0.2 Ma) and the José Fernandes Gabbro (134.5 ± 0.1 Ma). A 40Ar/39Ar phlogopite step-heating age of 133.7 ± 0.1 Ma from a lamprophyre dyke is identical within error to the U-Pb age of the diabase dykes, indicating that tholeiitic and alkaline magmatism were coeval in the Ponta Grossa Arch. Although nearly all analysed fractions are concordant and show low analytical uncertainties (± 0.3-0.9 Ma for baddeleyite; 0.1-0.4 Ma for zircon; 2σ), Pb loss is observed in all baddeleyite fractions and in some initial zircon fractions not submitted to the most extreme chemical abrasion treatment. The resulting age spread may reflect intense and continued magmatic activity in the Ponta Grossa Arch.

Permian single crystal U-Pb zircon age of the Rožňava Formation volcanites (Southern Gemeric Unit, Western Carpathians, Slovakia)

NASA Astrophysics Data System (ADS)

Vozárová, Anna; Šmelko, Miloš; Paderin, Ilya

2009-12-01

Zircon populations from the Rožňava Formation volcanic rock complex have been analysed. Euhedral zircons from the 1st volcanogenic horizon with fine oscillatory growth zoning, typical of magmatic origin, gave the average concordia age of 273.3 ± 2.8 Ma, with Th/U ratios in the range of 0.44-0.73. The Permian ages ranging from 266 to 284 Ma were identified in the wider, zoned or unzoned, central zircon parts, as well as in their fine-zoned oscillatory rims. The average concordia age of 275.3 ± 2.9 was obtained from the euhedral zircon population of the 2nd volcanogenic horizon of the Rožňava Formation. The analyses were performed on zoned magmatic zircons in the age interval from 267 to 287 Ma, with Th/U ratios in the range of 0.39-0.75. In the later zircon population two inherited zircon grains were dated giving the age of 842 ± 12 Ma (Neoproterozoic) and 456 ± 7 Ma (Late Ordovician). The magmatic zircon ages document the Kungurian age of Permian volcanic activity and contemporaneous establishment of the south-Gemeric basin. The time span of volcanic activity corresponds to the collapse of the Western Carpathian Variscan foreland which expanded southward.

Ar-40/Ar-39 and U-Th-Pb dating of separated clasts from the Abee E4 chondrite

NASA Technical Reports Server (NTRS)

Bogard, D. D.; Unruh, D. M.; Tatsumoto, M.

1983-01-01

Ar-40/Ar-39 and U-Th-Pb are investigated for three clasts from the Abee (E4) enstatite chondrite, yielding Ar-40/Ar-39 plateau ages (and/or maximum ages) of 4.5 Gy, while two of the clasts give average ages of 4.4 Gy. The 4.4-4.5 Gy range does not resolve possible age differences among the clasts. The U-Th-Pb data are consistent with the interpretation that initial clast formation occurred 4.58 Gy ago, and that the clasts have since remained closed systems which have been contaminated with terrestrial Pb. The thermal history of Abee deduced from Ar data seems consistent with that deduced from magnetic data, suggesting that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, experiencing no significant subsequent heating.

LA-ICP-MS and SIMS U-Pb and U-Th zircon geochronological data of Late Pleistocene lava domes of the Ciomadul Volcanic Dome Complex (Eastern Carpathians).

PubMed

Lukács, Réka; Guillong, Marcel; Schmitt, Axel K; Molnár, Kata; Bachmann, Olivier; Harangi, Szabolcs

2018-06-01

This article provides laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and secondary ionization mass spectrometry (SIMS) U-Pb and U-Th zircon dates for crystals separated from Late Pleistocene dacitic lava dome rocks of the Ciomadul Volcanic Dome Complex (Eastern Carpathians, Romania). The analyses were performed on unpolished zircon prism faces (termed rim analyses) and on crystal interiors exposed through mechanical grinding an polishing (interior analyses). 206 Pb/ 238 U ages are corrected for Th-disequilibrium based on published and calculated distribution coefficients for U and Th using average whole-rock and individually analyzed zircon compositions. The data presented in this article were used for the Th-disequilibrium correction of (U-Th)/He zircon geochronology data in the research article entitled "The onset of the volcanism in the Ciomadul Volcanic Dome Complex (Eastern Carpathians): eruption chronology and magma type variation" (Molnár et al., 2018) [1].

The age of universe

NASA Astrophysics Data System (ADS)

Ali, Zeeshan

The presence of short-lived isotope Curium-247 in the early Solar System complicates the job of dating the earliest events in the solar nebula. Primitive components in meteorites contain a detailed record of the conditions and processes in the solarnebula, the cloud of dust and gas surrounding the infant Sun. Determining accurately when the first materialsformed re-quires the lead-lead (Pb-Pb) dating method, a method based on the decay of uranium (U) isotopes toPb isotopes. The initial ratio of U-238 to U-235 is critical to determining theages correctly, and many studies have concluded that the ratio is constant for any given age. How-ever, my colleagues at Arizona State University(Frankfurt, Germany), and the Senckenberg Forschungsinstitut und Naturmuseum (also in Frankfurt) and I have found that some calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites deviate from the conventional value for the U-238/U-235 ratio. This could lead to inaccuracies of up to 5 million years in the age of these objects, if no correction is made.Variations in the concentrations of thorium and neodymium with the U-238/U-235 ratio suggest that the ratio may have been lowered by the decay of curium-247, which decays to U-235 with a half-life of 15.6 million years. Curium-247 is created in certain types of energetic supernovae, so its presence suggests that a supernova added material to the pre-solar interstellar cloud between 110 and 140 million years before theSolar System began to form.

Lifetime of an ocean island volcano feeder zone: constraints from U-Pb dating on coexisting zircon and baddeleyite, and 40/39Ar age determinations, Fuerteventura, Canary Islands

USGS Publications Warehouse

Allibon, James; Ovtcharova, Maria; Bussy, Francois; Cosca, Michael; Schaltegger, Urs; Bussien, Denise; Lewin, Eric

2011-01-01

High-precision isotope dilution - thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699-988 °C) to amphibole (500-600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520,000 to 800,000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Neodymium Isotopic Compositions of the Titanite Reference Materials Used in U-Pb Geochronology

NASA Astrophysics Data System (ADS)

Ma, Q.; Yang, Y.; Zhao, Z.

2017-12-01

Titanite (CaTiSiO5) is a widespread mineral and preferentially incorporates considerable uranium and significant light rare earth elements (LREEs) in its structure. Geochronology based upon U-Pb and Pb-Pb analyses of titanite has proven to be useful for understanding the P-T-t evolution of many igneous, metamorphic and hydrothermally altered rock samples (Scott and St-Onge, 1995). In the meantime, Sm-Nd isotopic composition in single titanite can be used to obtain initial Nd isotope composition at the time of titanite crystallization when combined with its U-Pb age, making titanite the most versatile mineral for dating metamorphism and tracing hydrothermal source (Amelin et al., 2009). The widely utilized in situ analyses by SIMS and LA-(MC)-ICP-MS have emphasized the significance for uniform and homogeneous reference materials for external correction (Liu et al., 2012, Sun et al., 2012, Yang et al., 2014). Here, we present U-Pb ages and Sm-Nd isotope analyses of twelve natural titanite crystals (12YQ82, T004, Ontario, BLR-1, OLT1, Khan, Qinghu, TLS-36, NW-IOA, C253, Pakistan and MKED1) acquired by Agilent 7500a Q-ICP-MS and Neptune MC-ICP-MS, respectively, combined a 193 nm ArF excimer laser ablation system. For U-Pb dating, elemental fractionation and instrumental drift were externally corrected using MKED1 titanite standard, showing results of U-Pb analyses all within error of those recommended values. With respect to Sm-Nd isotopes, we employed the interference-free 147Sm/149Sm to deduct 144Sm isobaric interference on 144Nd, and the fractionation between 147Sm and 144Nd was calibrated using BLR-1 titanite, which is proved homogenous in Sm-Nd isotopic system. The obtained Sm-Nd isotopic compositions for natural titanite samples are all consistent with those values determined by isotope dilution (ID) MC-ICP-MS, demonstrating the precision and accuracy currently available for in situ Sm-Nd analyses. Our results demonstrate that BLR-1, OLT1 and Ontario titanites are relatively uniform and homogeneous in Sm-Nd isotopic system and thus can serve as ideal reference materials for external calibration during in situ Sm-Nd analysis.

Earth Processes: Reading the Isotopic Code

NASA Astrophysics Data System (ADS)

Basu, Asish; Hart, Stan

Publication of this monograph will coincide, to a precision of a few per mil, with the centenary of Henri Becquerel's discovery of "radiations actives" (C. R. Acad. Sci., Feb. 24, 1896). In 1896 the Earth was only 40 million years old according to Lord Kelvin. Eleven years later, Boltwood had pushed the Earth's age past 2000 million years, based on the first U/Pb chemical dating results. In exciting progression came discovery of isotopes by J. J. Thomson in 1912, invention of the mass spectrometer by Dempster (1918) and Aston (1919), the first measurement of the isotopic composition of Pb (Aston, 1927) and the final approach, using Pb-Pb isotopic dating, to the correct age of the Earth: close—2.9 Ga (Gerling, 1942), closer—3.0 Ga (Holmes, 1949) and closest—4.50 Ga (Patterson, Tilton and Inghram, 1953).

Zircon Zoning, Trace Elements and U-Pb Dates Reveal Crustal Foundering Beneath the Pamir

NASA Astrophysics Data System (ADS)

Hacker, B. R.; Shaffer, M. E. F.; Ratschbacher, L.; Kylander-Clark, A. R.

2017-12-01

Xenoliths that erupted in the SE Pamir of Tajikistan at 11.2 Ma from 1000-1050°C and 90 km depth illuminate what happens when crust founders into the mantle. The xenoliths are a broad range of crustal rock types and contain abundant xenoliths whose U-Pb isotopic ratios and trace-element contents were examined by laser-ablation split stream inductively coupled plasma mass spectrometry. Cathodoluminescence imaging of the grains shows igneous cores with oscillatory zoning overprinted by substantial recrystallization. The bulk of the U-Pb dates are concordant and range from 160 Ma to 11 Ma. The range of dates suggest that the xenoliths were likely derived from the Jurassic-Cretaceous Andean-style magmatic arc and its Proterozoic-Mesozoic host rocks along the southern margin of Asia. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations that are interpreted to indicate garnet growth and minimal heating at 22-20 Ma, and then 200-300°C of heating, 25 km of burial, and alkali-carbonate melt injection at 14-11 Ma. These changes are interpreted to coincide with: i) heat input due to Indian slab breakoff at 22‒20 Ma; ii) rapid thickening and foundering of the Pamir lithosphere at 14‒11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

U-Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

NASA Astrophysics Data System (ADS)

Kalleson, E.; Corfu, F.; Dypvik, H.

2009-05-01

Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U-Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U-Pb data showing moderate discordance (5-13%) and indicating formation ages mostly in the range of 1600-1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995-999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14-40%) U-Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.

U-Pb thermochronology of the lower crust: producing a long-term record of craton thermal evolution

NASA Astrophysics Data System (ADS)

Blackburn, T.; Bowring, S. A.; Mahan, K. H.; Perron, T.; Schoene, B.; Dudas, F. O.

2010-12-01

The EarthScope initiative is focused on providing an enhanced view of the North American lithosphere and the present day stress field of the North American continent. Of key interest is the interaction between convecting asthenosphere and the conducting lithospheric mantle that underlie the continents, especially the cold ‘keels’ that underlie Archean domains. Cratonic regions are in general characterized by minimal erosion and or sediment accumulation. The Integration of seismic tomography, and mantle xenolith studies reveal a keel of seismically fast and relatively buoyant and viscous mantle; physical properties that are intimately linked with the long-term stability and topographic expression of the region. Missing from this model of the continental lithosphere is the 4th dimension--time--and along with it our understanding of the long-term evolution of these stable continental interiors. Here we present a thermal record from the North American craton using U-Pb thermochronology of lower crustal xenoliths. The use of temperature sensitive dates on lower crustal samples can produce a unique time-temperature record for a well-insulated and slowly cooling lithosphere. The base of the crust is insulated enough to remain unperturbed by any plausible changes to surface topography, yet unlike the subadjacent lithospheric mantle, contains accessory phases amenable to U-Pb dating (rutile, apatite, titanite). With near steady state temperatures in the lower crust between 400-600 °C, U-Pb thermochronometers with similar average closure temperatures for Pb are perfectly suited to record the long-term cooling of the lithosphere. Xenoliths from multiple depths, and across the craton yield time-temperature paths produced from U-Pb thermochronometers that record extremely slow cooling (<0.25 °C/Ma) over time scales of billions of years. Combining these data with numerical thermal modeling allow constraints to be placed on the dominant heat transfer mechanisms operating within the lithosphere including exhumation, conduction, decay of heat producing elements and thickness of crustal layers/lithospheric mantle. The thermal histories produced from this numerical model can in turn be used to calculate model U-Pb thermochronometric data using a numerical solution to the diffusion/production equation. Integration of thermal and volume diffusion models for the U-Pb system suggests that the extreme slow cooling recorded by U-Pb thermochronology is consistent with low integrated exhumation rates (<0.005 km/Ma). This exhumation rate is integrated over time-scales of hundreds of million to a billion years and does not preclude the possibility for rapid or short-wave length uplift/subsidence. This long-term record of continental lithosphere stability and apparent neutral buoyancy of the craton within a cooling mantle may be further used to refine our estimates of secular cooling within the mantle.

GHR1 - A new Eocene natural reference material for U-Pb and Hf isotopic measurements in zircon

NASA Astrophysics Data System (ADS)

Ibanez-Mejia, M.; Eddy, M. P.

2017-12-01

We present chemical abrasion-isotope dilution-thermal ionization (CA-ID-TIMS) U-Pb zircon geochronology and solution multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) Hf isotopic data from a proposed natural zircon reference material for use during in situ analyses of U-Pb and Hf isotopic ratios. The sample, GHR1, was collected from the rapakivi intrusive phase of the Eocene Golden Horn batholith in Washington, USA. Zircons separated from this sample range up to 250-300 μm in length and have moderate aspect ratios. A weighted mean of 15 Th-corrected 206Pb/238U zircon dates from GHR1 produced at the Massachusetts Institute of Technology is 48.132 ± 0.023 Ma (2σ analytical and tracer uncertainties only, MSWD=1.70) confirming that there is little or no inter-crystal age heterogeneity at the scale of a few 10 kyr. Solution MC-ICP-MS measurements of chemically purified aliquots give a 176Hf/177Hf weighted mean of 0.283050 ± 17 (2σ, n=10), corresponding to a ɛHf0 of ca. +9.3. The 2σ variability of these measurements is comparable to our reproducibility of the JMC-475 Hf isotopic standard 0.282160 ± 14 (n= 13), suggesting that GHR1 zircons are homogenous with respect to 176Hf/177Hf. In situ 206Pb/238U dates from collaborating secondary ion mass spectrometry (SIMS), sensitive high-resolution ion microprobe (SHRIMP), and laser ablation ICP-MS (LA-ICP-MS) laboratories are in excellent agreement with the CA-ID-TIMS date and illustrate the reproducibility and potential value of this reference zircon. The mean values of 176Hf/177Hf measurements from two LA-ICP-MS laboratories are in agreement with the solution MC-ICP-MS value, but show slightly greater dispersion and higher (Lu+Yb)/Hf values. We attribute this discrepancy to apatite inclusions that are high in REE and may lead to greater isobaric interferences on 176Hf. These inclusions and potential isobaric interferences from REE were removed during the chemical abrasion step prior to bulk dissolution and ion-exchange purification in the solution data. Nevertheless, the apparent isotopic homogeneity and reproducibility of 206Pb/238U and 176Hf/177Hf ratios and the potentially unlimited reserves of GHR1 suggest that it is a promising reference material. We plan to distribute GHR1 at the meeting to interested laboratories.

Application of U-Pb ID-TIMS dating to the end-Triassic global crisis: testing the limits on precision and accuracy in a multidisciplinary whodunnit (Invited)

NASA Astrophysics Data System (ADS)

Schoene, B.; Schaltegger, U.; Guex, J.; Bartolini, A.

2010-12-01

The ca. 201.4 Ma Triassic-Jurassic boundary is characterized by one of the most devastating mass-extinctions in Earth history, subsequent biologic radiation, rapid carbon cycle disturbances and enormous flood basalt volcanism (Central Atlantic Magmatic Province - CAMP). Considerable uncertainty remains regarding the temporal and causal relationship between these events though this link is important for understanding global environmental change under extreme stresses. We present ID-TIMS U-Pb zircon geochronology on volcanic ash beds from two marine sections that span the Triassic-Jurassic boundary and from the CAMP in North America. To compare the timing of the extinction with the onset of the CAMP, we assess the precision and accuracy of ID-TIMS U-Pb zircon geochronology by exploring random and systematic uncertainties, reproducibility, open-system behavior, and pre-eruptive crystallization of zircon. We find that U-Pb ID-TIMS dates on single zircons can be internally and externally reproducible at 0.05% of the age, consistent with recent experiments coordinated through the EARTHTIME network. Increased precision combined with methods alleviating Pb-loss in zircon reveals that these ash beds contain zircon that crystallized between 10^5 and 10^6 years prior to eruption. Mineral dates older than eruption ages are prone to affect all geochronologic methods and therefore new tools exploring this form of “geologic uncertainty” will lead to better time constraints for ash bed deposition. In an effort to understand zircon dates within the framework of a magmatic system, we analyzed zircon trace elements by solution ICPMS for the same volume of zircon dated by ID-TIMS. In one example we argue that zircon trace element patterns as a function of time result from a mix of xeno-, ante-, and autocrystic zircons in the ash bed, and approximate eruption age with the youngest zircon date. In a contrasting example from a suite of Cretaceous andesites, zircon trace elements record crystallization in a differentiating magma over ~200 ka. This new tool can lead to more robust ages for the deposition of volcanic rocks and therefore aids in calibrating the stratigraphic record. Our new U-Pb geochronology shows that the end-Triassic extinction and a concomitant negative spike in organic carbon isotopes and sea level occurred in far less than 300 ka and that these events coincided within 150 ka to the onset of CAMP volcanism in North America. These data substantially tighten the correlation between extinction and the CAMP, though more high-precision age constraints from sections that record these events, coupled with ongoing bio-, chemo- and magnetostratigraphic efforts, are needed to substantiate and understand causality. The post extinction recovery in ammonite diversity occurred in 2.0±0.2 Ma, coinciding with the duration of the Hettangian stage. Recovery of numerous ash beds from within the Hettangian allow determination of the absolute rates of ammonite diversification at the sub-zone level, constraining the interaction between volcanism, animal evolution and global environment in the wake of catastrophe.

Interpreting U-Pb data from primary and secondary features in lunar zircon

NASA Astrophysics Data System (ADS)

Grange, M. L.; Pidgeon, R. T.; Nemchin, A. A.; Timms, N. E.; Meyer, C.

2013-01-01

In this paper, we describe primary and secondary microstructures and textural characteristics found in lunar zircon and discuss the relationships between these features and the zircon U-Pb isotopic systems and the significance of these features for understanding lunar processes. Lunar zircons can be classified according to: (i) textural relationships between zircon and surrounding minerals in the host breccias, (ii) the internal microstructures of the zircon grains as identified by optical microscopy, cathodoluminescence (CL) imaging and electron backscattered diffraction (EBSD) mapping and (iii) results of in situ ion microprobe analyses of the Th-U-Pb isotopic systems. Primary zircon can occur as part of a cogenetic mineral assemblage (lithic clast) or as an individual mineral clast and is unzoned, or has sector and/or oscillatory zoning. The age of primary zircon is obtained when multiple ion microprobe analyses across the polished surface of the grain give reproducible and essentially concordant data. A secondary set of microstructures, superimposed on primary zircon, include localised recrystallised domains, localised amorphous domains, crystal-plastic deformation, planar deformation features and fractures, and are associated with impact processes. The first two secondary microstructures often yield internally consistent and close to concordant U-Pb ages that we interpret as dating impact events. Others secondary microstructures such as planar deformation features, crystal-plastic deformation and micro-fractures can provide channels for Pb diffusion and result in partial resetting of the U-Pb isotopic systems.

40 Ma of hydrothermal W mineralization during the Variscan orogenic evolution of the French Massif Central revealed by U-Pb dating of wolframite

NASA Astrophysics Data System (ADS)

Harlaux, Matthieu; Romer, Rolf L.; Mercadier, Julien; Morlot, Christophe; Marignac, Christian; Cuney, Michel

2018-01-01

We present U-Pb thermal ionization mass spectrometer (TIMS) ages of wolframite from several granite-related hydrothermal W±Sn deposits in the French Massif Central (FMC) located in the internal zone of the Variscan belt. The studied wolframite samples are characterized by variable U and Pb contents (typically <10 ppm) and show significant variations in their radiogenic Pb isotopic compositions. The obtained U-Pb ages define three distinct geochronological groups related to three contrasting geodynamic settings: (i) Visean to Namurian mineralization (333-327 Ma) coeval with syn-orogenic compression and emplacement of large peraluminous leucogranites (ca. 335-325 Ma), (ii) Namurian to Westphalian mineralization (317-315 Ma) synchronous with the onset of late-orogenic extension and emplacement of syn-tectonic granites (ca. 315-310 Ma) and (iii) Stephanian to Permian mineralization (298-274 Ma) formed during post-orogenic extension contemporaneous with the Permian volcanism in the entire Variscan belt. The youngest ages (276-274 Ma) likely reflect the reopening of the U-Pb isotopic system after wolframite crystallization and may correspond to late hydrothermal alteration (e.g. ferberitization). Our results demonstrate that W(±Sn) mineralization in the FMC formed during at least three distinct hydrothermal events in different tectono-metamorphic settings over a time range of 40 Ma.

Comparing pre- and post-chemical abrasion ages for Miocene Peach Springs Tuff zircon from ID-TIMS and SIMS analyses

NASA Astrophysics Data System (ADS)

Lidzbarski, M. I.; Mundil, R.; Miller, J. S.; Vazquez, J. A.

2012-12-01

The Miocene Peach Spring Tuff (PST) is a voluminous (>600 km3), zoned ignimbrite (trachyte to high-SiO2 rhyolite) that is exposed widely in eastern California, western Arizona, and southernmost Nevada, which was erupted from the Silver Creek caldera in the southwestern Black Mountains, AZ. PST serves as a regionally widespread marker unit and its eruption age has been determined to 18.8 to 18.9 Ma by 40Ar/39Ar methods, when corrected for systematic bias and normalized to the U-Pb system (Renne et al., 2010,). We performed ion-microprobe (SIMS) U-Pb dating of zircon from individual pumice clasts from PST to evaluate the growth history of zircon in the PST magma system. Sectioned, polished zircon from conventional epoxy mounts allows dating of internal growth domains (e.g. cores, interiors, and near-rim), whereas mounting unpolished zircon in indium and analyzing unpolished crystal faces provides a means to selectively sample the final increments of crystal growth (Reid and Coath, 2000). Combining U-Pb ages of unpolished zircon rims with near-rim interior analyses on sectioned grains yields a mean age of ca. 18.3 Ma, whereas ages of cores of sectioned crystals yield a mean of ca. 18.9 Ma. Several zircons have rim and/or core ages that are several hundred thousand years older or younger than these means (up to 1 m.y. total spread), although the uncertainties for individual SIMS ages are 2 to 5% (2 sigma uncertainty). Therefore, the distribution of ages is challenging to resolve. A modest number of the older grains are plausibly recycled antecrysts, but we suspect that the youngest zircons may have experienced Pb-loss. Failure to account for the possibility of inheritance and Pb-loss may lead to erroneous interpretations about crystallization in the PST system. In order to evaluate and mitigate the effects of Pb-loss, we employed the chemical abrasion (CA) technique of Mattinson (2005), which effectively eliminates domains in zircon that have suffered Pb-loss, and removes micro-inclusions that typically contain common Pb. Thermal annealing followed by CA techniques were used for ID-TIMS dating of a sub-set of zircon crystals previously analyzed by SIMS. Prior to TIMS analyses, zircon crystals were imaged by scanning electron microscopy (SEM) to evaluate the effects of CA on crystal domains sampled by SIMS. SEM images reveal that whole portions of crystals were removed by the CA technique, and a heterogeneous pattern of etching that was not confined to specific compositional zones visible in cathodoluminescence. Most of the SIMS sputter pits that yield spurious ages, are associated with etching and/or preferential annealing by the combined annealing and CA technique, suggesting that the young ages relative to the 40Ar/39Ar age may be due to Pb loss. ID-TIMS yields a coherent U-Pb age population of 18.8 Ma, with several older and younger crystals that might reflect xenocrysts, Pb-loss, and/or younger crystallization. In order to maintain spatial resolution and further evaluate the effects of Pb-loss in PST zircon, the annealing and CA-technique will be applied to zircon prior to SIMS dating. References: Reid and Coath, 2000, Geology 28: 443 Renne et al., 2010, GCA 78: 5349

Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry

USGS Publications Warehouse

Kingsbury, J.A.; Miller, C.F.; Wooden, J.L.; Harrison, T.M.

1993-01-01

Studies of the paragenesis and U-Pb systematics of monazite in rocks from the eastern Mojave Desert, California, corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance. Unmetamorphosed Latham Shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition. Monazite is absent in the shale and low-grade schists, but it is abundant in schists at staurolite and higher grades. Lower-grade schists instead include minute Th- and Ce-oxides and unidentified Ce-poor LREE-phosphates that apparently are lower-temperature precursors to monazite. Thus monazite originates when the pelite passes through lower-amphibolite-facies conditions. Monazites from three Upper Cretaceous granites yield ages that are strongly discordant. Upper intercepts of 1.6-1.7 Ga are similar to those defined by U-Pb data for coexisting zircons and coincide with a period of copious magmatism in the Mojave crust. As the host Upper Cretaceous granitic magmas were all above 700??C, effective closure of the restitic monazites to Pb loss must be well in excess of this temperature. U-Pb compositions of monazite from Proterozoic granitoids and schist also indicate high Pb retentivity. Taken together, these studies support the suggestion that monazite can be an effective prograde thermochronometer. At least in pelites, it is not usually retained as a detrital mineral, but rather forms during moderate-temperature metamorphism. Its U-Pb system should not be reset by subsequent higher-grade metamorphism. ?? 1993.

Chemical-abrasion SIMS dating of zircon from the Eocene Caetano caldera, Nevada

NASA Astrophysics Data System (ADS)

Colgan, J.; Watts, K. E.; John, D. A.; Henry, C. D.; Coble, M. A.; Vazquez, J. A.

2012-12-01

The Eocene Caetano caldera in northern Nevada formed during eruption of ~1100 km3 of crystal-rich rhyolite. Miocene extension cut the caldera into a set of fault blocks that expose minor pre-caldera volcanic rocks, two units of intracaldera Caetano Tuff up to 4 km thick, ash-flow tuff feeder dikes and ring-fracture intrusions, caldera collapse breccias, and post-collapse resurgent intrusions. Single-crystal 40Ar/39Ar sanidine dates on all parts of the caldera system overlap, yielding a 34.01 ± 0.05 Ma (n=17, Fish Canyon sanidine = 28.201 Ma) age for the eruption. 40Ar/39Ar dating also documents several preceding episodes of magmatism: 35.69 ± 0.06 Ma (sanidine, n =13) rhyolite dikes in the nearby Cortez gold district, 35.21 ± 0.18 Ma (plagioclase, n=1) andesite lava underlying Caetano Tuff, and a 38.90 ± 0.11 Ma (biotite, n=1), dacite dike in the northeastern caldera wall. Extensive U-Pb SHRIMP dating of zircon from both the Cortez dikes and all phases of the Caetano system suggests continuous magmatism from 40-34 Ma. However, all samples contain at least some—sometimes many—zircons with U-Pb ages younger than the 34.0 Ma argon age. To determine if anomalously young zircon ages are due to Pb-loss, we analyzed representative samples of the upper Caetano Tuff and the Redrock Canyon resurgent pluton with and without chemical abrasion to mitigate Pb-loss. Bulk zircon separates were annealed at 850°C for 48 hours, then chemically abraded with 10:1 HF/HNO3 vapor in a Parr bomb at 225°C for 8 hours, based on protocols outlined by Mattinson (2005). Both treated and untreated zircons from the same sample were mounted in epoxy and polished to their midsections, then imaged on the SEM using BSE and CL. The SHRIMP-RG at Stanford University was used to determine U-Pb ages and trace element concentrations in single spots for ~25 to 30 individual zircons per sample, using a round-robin procedure and two zircon age standards (R33 and 080) to monitor external precision. Analyses revealed distinctly different age populations for the abraded and untreated zircons. The chemically abraded populations yielded unimodal zircon age distributions with mean ages that overlap with the 40Ar/39Ar age. Untreated zircon populations yielded mean ages 0.9-1.5 Ma younger than the 40Ar/39Ar. In the untreated populations, 50-60% of zircon ages are younger than 34.0 Ma at 1σ, versus 15-20% in the chemically abraded populations. Comparison of trace element data from treated and untreated populations indicates that trace element concentrations are apparently unaffected by the chemical abrasion procedure. Further experiments are underway, but we tentatively conclude that chemical abrasion is effective for removing damaged Pb-loss portions of zircons while still enabling high spatial resolution U-Pb dating and trace element analysis. It appears to be a relatively fast and low-cost way to improve the accuracy of SIMS dating of large populations of zircon from Tertiary and older plutonic and volcanic rocks where Pb-loss is frequently an issue.

U-Pb geochronology and Hf-Nd isotope compositions of the oldest Neoproterozoic crust within the Cadomian orogen: new evidence for a unique juvenile terrane

NASA Astrophysics Data System (ADS)

Samson, S. D.; D'Lemos, R. S.; Blichert-Toft, J.; Vervoort, J.

2003-03-01

New U-Pb dates, combined with Nd and Hf isotopic data, from rocks within the Port Morvan area of the Baie de St Brieuc region of Brittany identify a unique portion of the Neoproterozoic Cadomia terrane. Two gneisses near Port Morvan yielded U-Pb dates of 754.6±0.8 Ma and 746.0±0.9 Ma, ages that are more than 130 Myr older than the oldest units formed during the main phase of early Cadomian magmatism. Two trondhjemite boulders from the monogenetic facies of the Cesson conglomerate yielded identical ages of 665.2±0.5 Ma and 665.5±0.7 Ma, and a cobble from the polygenetic facies yields a 207Pb- 206Pb date of 637±2 Ma. Individual detrital zircons from a sandstone associated with the Cesson conglomerates yield concordant U-Pb dates ranging from 650±3 Ma to 624.1±0.6 Ma. Initial ɛNd values for the rocks in this region range from +5.0 to +6.6, indicative of a substantial input from depleted mantle. Initial ɛHf values determined on zircons from these Neoproterozoic rocks, including the detrital zircons, range from +6.7 to +14.5, consistent with the Nd isotopic results. Maximum initial ɛHf values for two 2 Ga Icartian gneisses, considered basement to Cadomia, average +8.4 and +8.7. In contrast to the results of the Port Morvan rocks, 616-608 Ma syn-tectonic intrusions from Normandy and the British Channel Islands all have negative initial ɛNd values (-10.4 to -8.3) consistent with significant contamination by ancient crust such as the 2 Ga gneisses. The oldest arc-related magmas should have interacted most extensively with Cadomian basement, buffering younger mantle-derived magmas that were generated in subsequent magmatic episodes. The rocks within the Port Morvan region are thus inconsistent as examples of the earliest Cadomian intrusions as they show no evidence of interaction with 2 Ga basement. Instead, the older ages and mantle-like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to, and independently from, the Cadomian arc. Possible terrane-scale structural boundaries have recently been identified, including the newly recognized Port Morvan thrust fault and the NW-dipping Main Cadomian thrust.

Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on 0.1 Ma Toya Tephra, Japan" by Hisatoshi Ito

NASA Astrophysics Data System (ADS)

Guillong, M.; Schmitt, A. K.; Bachmann, O.

2015-04-01

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of eight zircon reference materials and synthetic zircon-hafnon end-members indicate that corrections for abundance sensitivity and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon. Other polyatomic interferences in the mass range 223-233 amu are insignificant. When corrected for abundance sensitivity and interferences, activity ratios of (230Th)/(238U) for the zircon reference materials we used average 1.001 ± 0.010 (1σ error; mean square of weighted deviates MSWD = 1.45; n = 8). This includes the 91500 and Plešovice zircons, which were deemed unsuitable for calibration of (230Th)/(238U) by Ito (2014). Uranium series zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th such as those presented by Ito (2014) are potentially unreliable.

Ediacaran ( 620 Ma) high grade regional metamorphism in the northern Arabian Nubian Shield: U/Th-Pb monazite ages of the Elat schist

NASA Astrophysics Data System (ADS)

Elisha, Bar; Katzir, Yaron; Kylander-Clark, Andrew

2017-04-01

Ediacaran times witnessed a hemisphere-scale orogenesis forming the extensive Pan-African mountain ranges and resulting in the final assembly of Gondwana supercontinent. The Elat metamorphic basement (S Israel) located at the northernmost tip of a major Pan-African orogenic suture, the Arabian Nubian Shield (ANS), comprises amphibolite facies schists and gneisses and was most likely shaped by this major continental collision. However the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are non-conclusive and a major emphasis was given to pre-Ediacaran island-arc related tectonics. This is mostly because U-Pb dating of zircon, widely used in Elat and elsewhere, is very successful in constraining the ages of the igneous and sedimentary protoliths, but is 'blind' to metamorphism at grades lower than granulite. Here U/Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat basement. Previous U-Pb dating of detrital zircon has shown that the sedimentary protoliths of the Elat schist are the oldest basement components (≥800 Ma), and detailed structural observations of the schists portrayed a complex deformation history including four successive phases (Shimron, 1972). The earliest three phases were defined as ductile and penetrative, but some of the available geochronological data apparently contradict field relations. In-situ analysis of metamorphic monazites by LASS (Laser Ablation Split Stream) involves simultaneous measurement of U/Th-Pb isotope ratios and REE contents in a single 10 μm sized grain or domain, thus allowing determining the age of specific texture and metamorphic assemblage. Monazite dating of the Elat schist yielded two concordant age clusters at 712±6 and 613±5 Ma. The corresponding REE patterns of the dated monazite grains indicate that porphyroblast growth, either garnet or staurolite, took place only during the younger event (M2). Likewise the regional south dipping penetrative foliation, common to the Elat schist and to all of the rocks of the Elat association, formed during the Ediacaran event (M2). This profound event started at 630 Ma and reached peak conditions of mid amphibolite facies at 620 Ma. Retrogression and stress relaxation shortly followed, involving overprint of staurolite schists by a cordierite-bearing assemblage at 613 Ma (M3), and was contemporaneous with the intrusion of andesitic dykes that were immediately metamorphosed to low-amphibolite. This metamorphic P-T-t path corresponds to the collision of East- and West-Gondwana as constrained by large goechronological database of post collision batholiths from all around the Arabian-Nubian Shield.

A test of uranium-series dating of fossil tooth enamel: results from Tournal Cave, France

USGS Publications Warehouse

Bischoff, J.L.; Rosenbauer, R.J.; Tavoso, A.; de Lumley, Henry

1988-01-01

A series of well preserved mammal bones and horse teeth was analyzed from archaeological levels of Tournal Cave (Magdalenian, Aurignacian, and Mousterain) to test the hypothesis that well-crystallized enamel behaves more as a closed system than does whole bone. The isotopic composition of bones and tooth enamels from this deposit meet criteria for confidence, and gave no reasons to suspect contamination or open-system behavior. Two samples for which 231Pa could be analyzed showed internal concordance with the respective 230Th ages. In spite of the favourable isotopic criteria, however, comparison of the U-series ages of the bones and the tooth enamel with stratigraphic position and 14C control indicated the dates were not meaningful. In general, both bones and tooth enamels gave ages too young, although some were clearly too old. Neither group showed any systematic increase of age with stratigraphic depth. Tooth enamel, therefore, shows no advantage over bone for U-series dating for this site. In Tournal cave both bones and enamel are apparently open to U, which is probably cycling as a consequences of post-depositional groundwater movement. ?? 1988.

TEMORA 1: A new zircon standard for Phanerozoic U-Pb geochronology

USGS Publications Warehouse

Black, L.P.; Kamo, S.L.; Allen, C.M.; Aleinikoff, J.N.; Davis, D.W.; Korsch, R.J.; Foudoulis, C.

2003-01-01

The role of the standard is critical to the derivation of reliable U-Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation- inductively coupled plasma-mass-spectrometry (ELA-ICP-MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/238U IDTIMS age has been determined to be 416.75??0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8??1.1 Ma for U-Pb intercomparisons between different laboratories that do not use a common spike. ?? 2003 Published by Elsevier Science B.V. All rights reserved.

Uranium isotope ratios of Muonionalusta troilite and complications for the absolute age of the IVA iron meteorite core

NASA Astrophysics Data System (ADS)

Brennecka, Gregory A.; Amelin, Yuri; Kleine, Thorsten

2018-05-01

The crystallization ages of planetary crustal material (given by basaltic meteorites) and planetary cores (given by iron meteorites) provide fiducial marks for the progress of planetary formation, and thus, the absolute ages of these objects fundamentally direct our knowledge and understanding of planet formation and evolution. The lone precise absolute age of planetary core material was previously obtained on troilite inclusions from the IVA iron meteorite Muonionalusta. This previously reported Pb-Pb age of 4565.3 ± 0.1 Ma-assuming a 238U/235U =137.88-only post-dated the start of the Solar System by approximately 2-3 million years, and mandated fast cooling of planetary core material. Since an accurate Pb-Pb age requires a known 238U/235U of the sample, we have measured both 238U/235U and Pb isotopic compositions of troilite inclusions from Muonionalusta. The measured 238U/235U of the samples range from ∼137.84 to as low as ∼137.22, however based on Pb and U systematics, terrestrial contamination appears pervasive and has affected samples to various extents for Pb and U. The cause of the relative 235U excess in one sample does not appear to be from terrestrial contamination or the decay of short-lived 247Cm, but is more likely from fractionation of U isotopes during metal-silicate separation during core formation, exacerbated by the extreme U depletion in the planetary core. Due to limited Pb isotopic variation and terrestrial disturbance, no samples of this study produced useful age information; however the clear divergence from the previously assumed 238U/235U of any troilite in Muonionalusta introduces substantial uncertainty to the previously reported absolute age of the sample without knowledge of the 238U/235U of the sample. Uncertainties associated with U isotope heterogeneity do not allow for definition of a robust age of solidification and cooling for the IVA core. However, one sample of this work-paired with previous work using short-lived radionuclides-suggests that the cooling age of the IVA core may be significantly younger than previously thought. This work indicates the metallic cores of protoplanetary bodies solidified no earlier than the first ∼5-10 million years of the Solar System.

Effective LA-ICP-MS dating of common-Pb bearing accessory minerals with new data reduction schemes in Iolite

NASA Astrophysics Data System (ADS)

Kamber, Balz S.; Chew, David M.; Petrus, Joseph A.

2014-05-01

Compared to non-destructive geochemical analyses, LA-ICP-MS consumes ca. 0.1 μm of material per ablation pulse. It is therefore to be expected that the combined analyses of ca. 200 pulses will encounter geochemical and isotopic complexities in all but the most perfect minerals. Experienced LA-ICP-MS analysts spot down-hole complexities and choose signal integration areas accordingly. In U-Pb geochronology, the task of signal integration choice is complex as the analyst wants to avoid areas of common Pb and Pb-loss and separate true (concordant) age complexity. Petrus and Kamber (2012) developed VizualAge as a tool for reducing and visualising, in real time, U-Pb geochronology data obtained by LA-ICP-MS as an add-on for the freely available U-Pb geochronology data reduction scheme of Paton et al. (2010) in Iolite. The most important feature of VizualAge is its ability to display a live concordia diagram, allowing users to inspect the data of a signal on a concordia diagram as the integration area it is being adjusted, thus providing immediate visual feedback regarding discordance, uncertainty, and common lead for different regions of the signal. It can also be used to construct histograms and probability distributions, standard and Tera-Wasserburg style concordia diagrams, as well as 3D U-Th-Pb and total U-Pb concordia diagrams. More recently, Chew et al. (2014) presented a new data reduction scheme (VizualAge_UcomPbine) with much improved common Pb correction functionality. Common Pb is a problem for many U-bearing accessory minerals and an under-appreciated difficulty is the potential presence of (possibly unevenly distributed) common Pb in calibration standards, introducing systematic inaccuracy into entire datasets. One key feature of the new method is that it can correct for variable amounts of common Pb in any U-Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems after common Pb correction. Common Pb correction can be undertaken using either the 204Pb, 207Pb or 208Pb(no Th) methods. After common Pb correction to the user-selected age standard integrations, the scheme fits session-wide model U-Pb fractionation curves to the time-resolved U-Pb standard data. This down hole fractionation model is next applied to the unknowns and sample-standard bracketing (using a user specified interpolation method) is used to calculate final isotopic ratios and ages. 204Pb- and 208Pb(no Th)-corrected concordia diagrams and 204Pb-, 207Pb- and 208Pb(no Th)-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline. Apatite, titanite, rutile and very young zircon data will be presented, obtained using a Thermo Scientific iCAP-Qc (Q-ICP-MS) coupled to a Photon Machines Analyte Excite 193 nm ArF Excimer laser with a novel signal smoothing device Chew, D.M., Petrus, J.A., and Kamber, B.S. (2014); Chemical Geology, 363, 185-199. Paton C., Woodhead J.D., Hellstrom J.C., Hergt J.M., Greig A. and Maas R. (2010); Geochemistry Geophysics Geosystems, 11, 1-36. Petrus, J.A. and Kamber, B.S. (2012): Geostandards and Geoanalytical Research, 36, 247-270.

Geological setting and timing of the cassiterite vein type mineralization of the Kalima area (Maniema, Democratic Republic of Congo)

NASA Astrophysics Data System (ADS)

Dewaele, S.; Muchez, Ph; Burgess, R.; Boyce, A.

2015-12-01

The Central African Mesoproterozoic Karagwe-Ankole belt in the Great Lakes area (DRCongo, Rwanda, Burundi, Uganda and Tanzania) forms a metallogenic province that hosts a variety of granite-related mineralization, which contains cassiterite, columbite-tantalite, wolframite/ferberite, spodumene and beryl. The Kalima area in the Maniema province of the DRCongo forms one of the most important areas for cassiterite mineralization in the eastern part of the DRCongo, even after many decades of exploitation. The mineralization dominantly consists of quartz veins that are hosted in Mesoproterozoic metasediments at the contact with granitic rocks of the Kalima granite (Avuanga and Yubuli) or directly crosscutting these granitic rocks (Atondo). Only limited - and mainly unmineralized pegmatites - have been described in the Lutshurukuru area. Mineralized quartz veins - and some granite bodies - intruded following the regional tectonic foliation or existing fracture zones, confirming the late-to post-tectonic origin of the fertile granite system. The emplacement of the quartz veins resulted in an alteration of the metasedimentary and granitic host-rocks, mainly resulting in muscovitization, tourmalinization and silicification. Cassiterite itself formed relatively late during vein formation and is associated with muscovite in fractures in or along the margins of the quartz veins. 40Ar-39Ar age dating of muscovite of an unmineralized pegmatite from the Lutshurukuru area gave an excellent plateau age of 1024 ± 5.5 Ma, while the muscovite associated with mineralization gave plateau ages of 986 ± 5.3 Ma for the Atondo deposit and 992.4 ± 5.4 Ma for the Yubuli deposit. The rather large spread in ages between the supposed parental granite/pegmatite and quartz veins is interpreted to reflect different magmatic events in the evolution of a composite granite system, starting at ∼1020 Ma and ending with mineralized quartz vein formation at ∼990 Ma. The latter age corresponds with the U-Pb age reported for columbite-tantalite in the area (993 ± 1 Ma at Kamisuku), which could be interpreted as the primary formation age of a new generation of mineralized pegmatites in the Kalima area, or as the resetting age of the U-Pb system during the ∼990 Ma mineralizing event. Muscovite of a mineralized greisen sample of Avuanga gave a plateau age with relaxed constraints of 1010.3 ± 5.9 Ma, which has been interpreted as a partially resetting of muscovite formed at ∼1020 Ma age, during the ∼990 Ma event.

Problems of geological and isotopic age of the Okhotsk-Chukotsk Volcanogenic Belt (OCVB)

NASA Astrophysics Data System (ADS)

Belyi, V. F.

2008-12-01

The working stratigraphic chart accepted for the Okhotsk-Chukotsk Volcanogenic Belt (OCVB) at the 3rd Interdepartmental Regional Stratigraphic Conference (IRSC) integrated data of the medium-scale geological survey, the established succession of endemic floras of the developing volcanic highland, and the results of palynological and magnetostratigraphic study ( Resolutions of the 3rd…, 2003). The OCVB was formed during the middle Albian-Santonian (and initial Campanian probably). Sequences of the belt are attributed to the Buor-Kemyus (early-middle Albian), Arman (late Albian), Amka (Cenomanian), and Arkagala (Turonian-Santonian) phytostratigraphic horizons. The lack of data on relations between the horizons and fauna-bearing marine deposits is a serious obstacle for correlation of regional subdivisions with the general stratigraphic scale. The problem can be solved using geological methods of palynological and tephrochronological research. Isotopic ages of the OCVB rocks were determined applying the K-Ar and Rb-Sr isotopic dating in the early period and the Ar-Ar and U-Pb (SHRIMP) methods in recent years. The subdivision scheme of the OCVB volcanics based on the K-Ar relict and Rb-Sr isochron dates, which is accepted as addendum to the working stratigraphic chart, confirms in general the geological inferences concerning the OCVB age. The Ar-Ar and U-Pb dates (less than 100 determinations in total) obtained for the Okhotsk, Central Chukotsk, and Anadyr sectors of the OCVB external zone provoked opinions that the belt age should be radically revised. Analysis of new isotopic dates showed that they contradict in variable extent to geological data on the Okhotsk and Central Chukotsk sectors, whereas there is no significant discordance between isotopic and geological data on the Anadyr sector. Consequently, it can be empirically concluded that geological factors influenced the isotopic systems (“clock”). There is also a considerable discordance between the Ar-Ar and U-Pb isotopic dates. These uncertainties of the isotopic dating imply prematurity of idea to revise age of the OCVB.

Provenance of Marine Sediment in the Gulf of Alaska, IODP Expedition 341: Links Between Sediment Derivation, Glacial Systems, and Exhumation of the Coastal Mountain Belts

NASA Astrophysics Data System (ADS)

Allen, W. K.; Dunn, C. A.; Enkelmann, E.; Ridgway, K.; Colliver, L.

2015-12-01

Provenance analysis of Neogene sand and diamict beds from marine boreholes drilled by the IODP Expedition 341 provides a marine sedimentary record of the interactions between tectonics, climate and sediment deposition along a glaciated convergent margin. The 341 boreholes represent a cross-margin transect that sampled the continental shelf, slope, and deep sea Surveyor Fan of the Gulf of Alaska. Our dataset currently consists of ~ 650 detrital zircons selected for double dating method utilizing both detrital zircon fission track (FT) and U-Pb analysis from sand and diamict beds, as well as zircon U-Pb geochronology and apatite FT from igneous and gneissic clasts. Detrital zircon U-Pb geochronology of sand records dominant peak ages of 53, 62, 70, and 98 Ma with minor populations of 117, 154, and 170 Ma. Most of these ages can be correlated to primary igneous sources in the Coast Plutonic Complex, the Chugach Metamorphic Complex, the plutonic rocks of Wrangellia, and the Sanak-Baranoff plutonic belt. All samples analyzed to date, covering a 10 Myr range, share nearly identical detrital zircon populations suggesting similar primary sediment sources and reworking of sediment in thrust belts and accretionary prisms along this convergent margin. Plutonic and gneissic clasts collected from the boreholes on the shelf have already been double dated. These clasts have general U-Pb zircon crystallization ages of 52-54 Ma and apatite fission track cooling ages of 10-12 Ma. These results, along with previous published studies, indicate that these clasts were derived from the Chugach Metamorphic Complex and were eroded and transported by the Bagley Ice Field and Bering Glacier. Future results using this approach should allow us to pinpoint which parts of the exhumed onshore ranges and which glacial systems provided sediment to marine environments in the Gulf of Alaska.

The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

USGS Publications Warehouse

Rioux, Matthew; Farmer, Lang; Bowring, Samuel; Wooton, Kathleen M.; Amato, Jeffrey M.; Coleman, Drew S.; Verplanck, Philip L.

2016-01-01

The Organ Mountains caldera and batholith expose the volcanic and epizonal plutonic record of an Eocene caldera complex. The caldera and batholith are well exposed, and extensive previous mapping and geochemical analyses have suggested a clear link between the volcanic and plutonic sections, making this an ideal location to study magmatic processes associated with caldera volcanism. Here we present high-precision thermal ionization mass spectrometry U–Pb zircon dates from throughout the caldera and batholith, and use these dates to test and improve existing petrogenetic models. The new dates indicate that Eocene volcanic and plutonic rocks in the Organ Mountains formed from ~44 to 34 Ma. The three largest caldera-related tuff units yielded weighted mean 206Pb/238U dates of 36.441 ± 0.020 Ma (Cueva Tuff), 36.259 ± 0.016 Ma (Achenback Park tuff), and 36.215 ± 0.016 Ma (Squaw Mountain tuff). An alkali feldspar granite, which is chemically similar to the erupted tuffs, yielded a synchronous weighted mean 206Pb/238U date of 36.259 ± 0.021 Ma. Weighted mean 206Pb/238U dates from the larger volume syenitic phase of the underlying Organ Needle pluton range from 36.130 ± 0.031 to 36.071 ± 0.012 Ma, and the youngest sample is 144 ± 20 to 188 ± 20 ka younger than the Squaw Mountain and Achenback Park tuffs, respectively. Younger plutonism in the batholith continued through at least 34.051 ± 0.029 Ma. We propose that the Achenback Park tuff, Squaw Mountain tuff, alkali feldspar granite and Organ Needle pluton formed from a single, long-lived magma chamber/mush zone. Early silicic magmas generated by partial melting of the lower crust rose to form an epizonal magma chamber. Underplating of the resulting mush zone led to partial melting and generation of a high-silica alkali feldspar granite cap, which erupted to form the tuffs. The deeper parts of the chamber underwent continued recharge and crystallization for 144 ± 20 ka after the final eruption. Calculated magmatic fluxes for the Organ Needle pluton range from 0.0006 to 0.0030 km3/year, in agreement with estimates from other well-studied plutons. The petrogenetic evolution proposed here may be common to many small-volume silicic volcanic systems.

Did the Kiruna iron ores form as a result of a metasomatic or igneous process? New U-Pb and Nd data for the iron oxide apatite ores and their host rocks in the Norrbotten region of northern Sweden

NASA Astrophysics Data System (ADS)

Westhues, A.; Hanchar, J. M.; Whitehouse, M. J.; Fisher, C. M.

2012-12-01

A number of iron deposits near Kiruna in the Norrbotten region of northern Sweden are of the iron oxide apatite (IOA) type of deposits; also referred to as Kiruna-type deposits. They are commonly considered a subgroup or end-member of iron oxide copper gold (IOCG) deposits, containing no economic grades of copper or gold. Both IOCG and IOA deposits are characterized by abundant low-Ti Fe oxides, an enrichment in REE, and intense sodium and potassium wall-rock alteration adjacent to the ores. Deposits of these types are of a great economic importance, not only for iron, but also for other elements such as rare earth elements (REE) or uranium. Kiruna, the type locality of the IOA type of mineral deposits, is the focus of this study. Despite a century-long mining history and 2500 Mt of iron ore produced in the region to date (with grades of 30 to 70 wt.% Fe), the genesis of these deposits is poorly understood: theories of a magmatic vs. a hydrothermal or metasomatic origin have been debated, and the timing of mineralization of the ores in the Norbotten region has never been directly dated. The results anticipated from this study will provide a better understanding of the nature of the IOA type of mineral deposits and their relation to IOCG deposits such as Olympic Dam in Australia. An array of geochemical methods is used in order to gain insights on the emplacement history of the host rocks, their subsequent alteration, and the ore genesis of these deposits. This includes in situ U/Pb geochronology of zircon, monazite, and titanite to constrain the timing between host rock emplacement, alteration and mineralization. Isotopic data from whole rocks and in situ at mineral scale will provide constraints on the involvement of hydrothermal fluids and their possible sources, as well as on the sources of Fe, U, and the REE. Newly obtained Sm-Nd isotopic data points to distinct source differences between host rocks, ore and alteration related samples. Preliminary in situ U-Pb dating of zircon from both host rock and ore samples confirms a previously documented event around 1880 - 1900 Ma in the Norrbotten region. However, U-Pb in monazite from an ore sample suggests a further event at ca. 1650 Ma, a period of known activity in Fennoscandia. Further investigation and more U-Pb data are needed to confirm those dates and how the iron mineralization is related to those two events. The combination of U-Th-Pb ages, tracer isotopes and trace element abundances at mineral scale (e.g., Lu-Hf in zircon, and Sm-Nd in monazite, apatite, titanite), along with the O isotopic composition of zircon, will be used to decipher whether the Kiruna iron ore deposits are of metasomatic or igneous origin. Overall, the study also intends to develop a predictive model for exploration of similar iron oxide apatite deposits worldwide.

Provenance implications of Th U Pb electron microprobe ages from detrital monazite in the Carboniferous Upper Silesia Coal Basin, Poland

NASA Astrophysics Data System (ADS)

Kusiak, Monika Agnieszka; Kędzior, Artur; Paszkowski, Mariusz; Suzuki, Kazuhiro; González-Álvarez, Ignacio; Wajsprych, Bolesław; Doktor, Marek

2006-05-01

This paper reports the results of CHIME (chemical Th-U-Pb isochron method) dating of detrital monazites from Carboniferous sandstones in the Upper Silesia Coal Basin (USCB). A total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence. Age distributions were identified in detrital monazites from the USCB sequence and correlated with specific dated domains in potential source areas. Most monazites in all samples yielded ca. 300-320 Ma (Variscan) ages; however, eo-Variscan, Caledonian and Cadomian ages were also obtained. The predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic Bohemian Massif (BM), which suggests that various crystalline lithologies in the BM were the dominant sources of USCB sediments.

Mercury anomalies as a proxy for large igneous province volicanism and effects on the carbon cycle in a U-Pb age-constrained section spanning the end-Triassic mass extinction, Levanto, Peru

NASA Astrophysics Data System (ADS)

Yager, J. A.; West, A. J.; Bergquist, B. A.; Thibodeau, A. M.; Corsetti, F. A.; Berelson, W.; Rosas, S.; Bottjer, D. J.

2017-12-01

Understanding the causes of the end-Triassic extinction and their potential relationship to Central Atlantic Magmatic Province (CAMP) volcanism necessitates careful correlation of carbon cycle records (largely from marine sections) and volcanism (largely from terrestrial successions) in a robust chronological framework. Here, we report stable carbon isotopes and mercury concentrations and isotopes from the Levanto section in Northern Peru as a putative proxy for CAMP (a large igneous province) in a marine section. Levanto represents deposition well below storm wave base and is lithologically homogenous before, during, and after the end-Triassic extinction interval, making it ideal for detailed chemostratigraphy. Furthermore, abundant intercalated ash beds allow us to correlate mercury concentrations in the marine record directly with CAMP basalt ages, providing a test of the correspondence of mercury anomalies with the eruption of CAMP volcanics. Age dating and C isotope analyses provide an opportunity to explore orbital tuning of the carbon isotope record and ground truth it with existing U-Pb ages from the section, a feature not available in any other marine sections examined to date. The abundance of U-Pb dated ashes in the Levanto section allows us to correlate orbital tuning with other basins, which lack absolute age control, providing a better understanding for the C cycle changes associated with the Triassic-Jurassic boundary.

Late Pleistocene granodiorite beneath Crater Lake caldera, Oregon, dated by ion microprobe

USGS Publications Warehouse

Bacon, C.R.; Persing, H.M.; Wooden, J.L.; Ireland, T.R.

2000-01-01

Variably melted granodiorite blocks ejected during the Holocene caldera-forming eruption of Mount Mazama were plucked from the walls of the climactic magma chamber ~15 km depth. Ion-microprobe U-Pb dating of zircons from two unmelted granodiorite blocks with SHRIMP RG (sensitive high-resolution ion microprobe-reverse geometry) gives a nominal 238U/206Pb age of 101+78-80 ka, or 174+89-115 ka when adjusted for an initial 230Th deficit. SHRIMP RG U-Th measurements on a subset of the zircons yield a 230Th/238U isochron age of 112 ?? 24 ka, considered to be the best estimate of the time of solidification of the pluton. These results suggest that the granodiorite is related to andesite and dacite of Mount Mazama and not to magmas of the climactic eruption. The unexposed granodiorite has an area of at least 28 km2. This young, shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions ~7700 yr ago, resulting in collapse of Crater Lake caldera.

Concordant ages for the Lava Creek Tuff from high-spatial-resolution U-Pb dating of zircon rim faces and single-crystal sanidine 40Ar/39Ar dating

NASA Astrophysics Data System (ADS)

Matthews, N. E.; Vazquez, J. A.; Calvert, A. T.

2013-12-01

The last great explosive supereruption from the Yellowstone Plateau formed present-day Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff (LCT). The LCT eruption blanketed much of the western United States in ash, and consequently is a key chronostratigraphic marker bed for delimiting Quaternary uplift rates, the age of middle Pleistocene glacial and pluvial deposits, and tephra correlation in North America. Previous 40Ar/39Ar dating of the two mineralogically distinct LCT members (A & B) yield ages ranging from ca. 600 ka (Gansecki et al., 1998) to ca. 640 ka (Lanphere et al., 2002). To resolve the timing of eruption and crystallization timescale for the LCT magma, we dated both LCT members using a dual-method approach as follows: (1) ion microprobe (SHRIMP-RG) U-Pb dating and trace-element characterization of the final few micrometers of zircon crystallization by analysis of unpolished rims on indium-mounted crystals, and dating of the onset of zircon crystallization by traditional analysis of sectioned crystal interiors, and (2) laser-fusion 40Ar/39Ar dating of single sanidine crystals from bulk LCT ignimbrite and pumice. The unpolished rims of zircon from LCT members A & B yield indistinguishable ages, with a mean age of 621.8 × 2.5 ka (1σ) after correction for initial 230Th disequilibrium as constrained by ion-probe analyses of LCT melt inclusions. Single sanidine crystals from LCT-B yield a mean age of 624.9 × 2.6 ka (FCT=28.17 Ma) that is indistinguishable from the zircon rim ages for both members. These results indicate that LCT members A & B erupted over a geologically brief interval, which is supported by the direct and gradational contact of their equivalent fallout in distal lacustrine deposits and a lack of field evidence for a significant time-break between the LCT A & B in proximal deposits (Christiansen, 2001), but contrasts with older Yellowstone ignimbrite (e.g., Huckleberry Ridge) that may have erupted as multiple pulses over tens of kyr (Ellis et al., 2012). Both LCT members contain zircon whose interiors yield U-Pb ages that are mostly ca. 40 kyr older than their rims, suggesting a short timescale for assembly of the LCT magma chamber relative to some other voluminous bodies of silicic magma (e.g., Fish Canyon). Trace-element concentrations in zircon from Members A and B appear diagnostic, with the former containing higher U and REE concentrations. Our concordant results from U-Pb and 40Ar/39Ar dating indicating a ca. 625 ka age for the LCT eruption are supported by the stratigraphic position of LCT-B tephra at or near the top of Great Basin pluvial lake bed sequences associated with Marine Isotope Stage 16, whose termination is astronomically dated at 621 ka. Christiansen, 2001, USGS Prof. Pap. 729-G; Ellis et al., 2012, Quat Geochron 9: 31-41; Gansecki et al., 1998, Geology 26(4): 343-346; Lanphere et al., 2002, GSA Bull. 114(5): 559-568.

Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions

USGS Publications Warehouse

Harlan, S.S.; Heaman, L.; LeCheminant, A.N.; Premo, W.R.

2003-01-01

Precise U-Pb baddeleyite dating of mafic igneous rocks provides evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. U-Pb baddeleyite analyses for eight intrusions from seven localities ranging from the northern Canadian Shield to northwestern Wyoming-southwestern Montana are statistically indistinguishable and yield a composite U-Pb concordia age for this event of 780.3 ?? 1.4 Ma (95% confidence level). This 780 Ma event is herein termed the Gunbarrel magmatic event. The mafic magmatism of the Gunbarrel event represents the largest mafic dike swarm yet identified along the Neoproterozoic margin of Laurentia. The origin of the mafic magmatism is not clear, but may be related to mantle-plume activity or upwelling asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto-Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by ???60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events provides unique time markers that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.

Quantifying the timescales of Archean UHT metamorphism through U-Pb monazite and zircon petrochronology

NASA Astrophysics Data System (ADS)

Guevara, V.; MacLennan, S. A.; Schoene, B.; Dragovic, B.; Caddick, M. J.; Kylander-Clark, A. R.; Couëslan, C. G.

2016-12-01

Unraveling the timescales of metamorphism is crucial to understanding the mechanisms behind mass/heat transfer through Earth's crust. Though such mechanisms and their durations are becoming well constrained in modern (Phanerozoic) settings, the drivers of metamorphism in the ancient geologic record remain more enigmatic. The development of accessory phase petrochronology has allowed metamorphic evolution to be closely linked to isotopic dates, ultimately improving quantification of metamorphic durations. While in-situ petrochronological methods preserve textural and spatial context, they often lack the temporal resolution required to accurately quantify metamorphic duration in Archean terranes. Here we combine in-situ U-Pb monazite (mnz) and zircon (zrn) laser ablation split-stream (LASS) and high-precision ID-TIMS-TEA petrochronology of distinct grain domains to resolve the timescales of ultrahigh temperature (UHT) metamorphism in the Archean Pikwitonei granulite domain (PGD). The PGD encompasses >1.5x105 km2 of granulite-facies rocks on the NW edge of the Superior Province. Themodynamic modelling of a pelite from the western part of the PGD suggests peak P-T conditions of >8 kbar, 900-940 °C and UHT decompression to 8 kbar followed by cooling. LASS analysis of zrn inclusions in garnet (grt) yields a date of 2701 Ma, with Ti in zrn thermometry yielding T of 800-900 °C. LASS analysis of mnz yields dates of 2720-2680 Ma for low HREE domains with no to shallow negative Eu anomalies, suggestive of growth during plagioclase (plg) breakdown and grt stability. ID-TIMS analysis of a mnz fragment with a strong negative Eu anomaly, suggestive of growth during plg stability, gives a concordant 207Pb/206Pb date of 2666 Ma, consistent with LASS results of 2660-2640 Ma for chemically similar domains. ID-TIMS analyses of zrn rims yield a range of 207Pb/206Pb dates from 2671 to 2656 Ma (±<1 Ma). Ti in zrn yields 800 °C for these rims, indicating they grew at similar T. Together, these data indicate a metamorphic cycle in the PGD to/from UHT over a minimum of 35 Ma, with at least 12 Ma of slow cooling near 800 °C in the lower crust following UHT decompression. This evolution is inconsistent with punctuated thermal pulses due to focused fluid flow or magmatism, instead requiring a long-lived source of crustal heating.

Textural and U-Pb systematics (CA-TIMS) of stepwise leaching in zircon from granophyres in the Archean Stillwater Complex

NASA Astrophysics Data System (ADS)

Wall, C. J.; Scoates, J. S.; Friedman, R. M.; Meurer, W. P.

2011-12-01

The chemical abrasion-TIMS method or CA-TIMS uses a high-temperature annealing treatment to remove the effects of Pb-loss from radiation damaged parts of the zircon lattice and allows for highly precise and accurate U-Pb dating [1]. Zircon with high U-Th concentrations can be strongly metamict and it is not yet clear how effective the chemical abrasion treatment is when applied to these types of grains. In this study, we evaluate the link between the textural response and U-Pb systematics of zircon during chemical leaching for a granophyric rock from the Archean Stillwater Complex in Montana. The sample was selected based on the high abundance of zircon and the high degree of metamictization of the grains. Untreated and leached zircon grains were analyzed by scanning electron microscopy (SEM) and isotope dilution thermal ionization mass spectrometry (ID-TIMS). In thin section, zircon grains are euhedral in morphology and tend to be associated with amphibole-rich zones. Under the SEM, zircon grains typically have two distinct zones, a Ca-rich amorphous zone in the core and a more intact outer shell. Five acid-leaching steps were carried out on grains with each step increasing in temperature and acid strength until the zircon residue completely dissolved (starting at a 50% strength HF/HNO3 mixture at 100°C for 4 hours and finishing at full strength acid at 170°C for 4 hours). SEM imaging was conducted on grains after each step with a noticeable change in the morphology of the grains. As the leaching progressed, the acid leach created large pathways through the crystal lattice until only grain fragments remained, in some cases even boring large holes into the centre of the zircon grain. The acid preferentially dissolved the more soluble Ca-rich zones leaving behind fragile zircon "shells". U-Pb results of untreated grains are highly discordant (37-80%) and yield a chord with an upper intercept age of 1981 ± 140 Ma, whereas leached grains are slightly less discordant (12-69%) and define a much different chord with an upper intercept age of 2618 ± 4 Ma. Both ages are too young in comparison to the crystallization age of the Stillwater Complex (ca. 2709 Ma) and are considered to represent two discrete Pb-loss events that have reset the U-Pb systematics of these zircon grains. These Pb-loss events may be due to hydrothermal events associated with the emplacement of mafic dikes or the crystallization of regionally extensive monzonite plutons that intruded the Stillwater Complex. Dating different geological events using the U-Pb isotopic compositions of untreated metamict zircon and leached zircon by systematic acid leaching could prove to be a useful application of CA-TIMS. [1] Mattinson (2005) Chemical Geology 220, 47-66.

Sharpening the U-Th Chronometer: Progress and Outlook

NASA Astrophysics Data System (ADS)

McLean, N. M.; Condon, D. J.; Henderson, G. M.; Richards, D. A.; Noble, S.; Mason, A.

2013-12-01

Uranium is incorporated into a variety of natural materials when they form, including carbonates like speleothems and corals. The two most abundant naturally occurring uranium isotopes, 238U and 235U, decay to 206Pb and 207Pb over long timescales with half-lives of 4.5 and 0.7 billion years respectively, but transition through several intermediate daughter isotopes with shorter half-lives first. Fractionation between these daughter isotopes, including 234U, 230Th, and 231Pa, and their parent isotopes, followed by their time-dependent return to secular equilibrium over the course of up to ~800 kyr, forms the basis for U-series geochronology, and allows speleothems and corals to be precisely dated. These carbonates often additionally incorporate chemical and isotopic signatures (e.g., trace elements, δ18O and δ13C) from the environment in which they form, and thus are some of the best dated paleoclimate archives, offering clues about past and future conditions for life on Earth. Over the past decade, the analytical precision of U-series isotope measurements has improved dramatically, largely due to the steadily increasing sensitivity of multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Analytical uncertainties in U-Th dates now approach or are better than 0.1% (2σ), for instance ×100 years for a speleothem or coral that is 130 kyr old (Cheng et al., 2013). However, the accuracy of U-series dates also depends on the accuracy of tracer calibrations, reference solutions and data reduction protocols, which has not kept pace in many laboratories. This means that dates measured in different labs, while impressively precise, may not be directly comparable. To address issues of inter-laboratory bias and improve the accuracy and inter-comparability of U-Th dates, we have instigated work in three related directions. First, we report on the mixing of three synthetic U-Th age solutions, created by combining high-purity mono-isotopic solutions to mimic the isotopic composition of commonly analyzed materials. These age solutions will form the basis for an inter-laboratory comparison and serve as international reference materials for assessing long-term reproducibility. Second is the creation of several U-Th gravimetric solutions, which can be used to calibrate the mixed 229Th-236U×233U tracers used in the community without having to rely on materials assumed to be in secular equilibrium. Tracers calibrated against these solutions, and therefore U-Th dates measured with these tracers, will be fully metrologically traceable. Finally, we report progress on and important considerations in developing data reduction and uncertainty propagation protocols for calculating U-Th dates and constructing and interpreting U-Th age models. Reference: Cheng et al. (2013) EPSL 371-372, 82-91

The Onset of the Sturtian Snowball Earth: New Geochronology and Chemostratigraphy from the Tambien Group of Ethiopia

NASA Astrophysics Data System (ADS)

Park, Y.; MacLennan, S. A.; Swanson-Hysell, N.; Maloof, A. C.; Schoene, B.; Alene, M.; Tremblay, M. M.; Anttila, E.; Haileab, B.

2016-12-01

The Snowball Earth hypothesis proposes that the onset of global glaciation was a rapid event, particularly at low-latitudes, due to a runaway ice-albedo instability in the climate system. Currently, the Neoproterozoic Sturtian Snowball Earth Glaciation is constrained to have been ongoing at low-latitudes in present day NW Canada at 716.47 ± 0.24 Ma based on a U-Pb date from a tuff within glacial diamictite (Macdonald et al., 2010). A date from a rhyolite underlying this diamicitite of 717.43 ± 0.14 Ma has been inferred to precede the glaciation, although this age is not a strict maximum for the initiation of the snowball Earth since evidence of glaciation may have been obscured in volcanic units. The Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence deposited in an arc proximal basin that conformably culminates in diamictite associated with the Sturtian Glaciation. This study exploits the presence of tuffs suitable for high precision U-Pb zircon geochronology interbedded with recently discovered exposures of the transition into the snowball Earth. Dates from tuffs stratigraphically <100 m below massive diamictite enable a test of synchronicity for the initiation of the Sturtian Glaciation when compared with data from NW Canada and S China. Furthermore, tuffs in the proximity of the chemostratigraphically defined Islay negative δ13C anomaly give the opportunity to develop new U-Pb dates to test the prediction from Re-Os dating of black shales in NW Canada that the Islay significantly predates the Sturtian ice age, decoupling the sharpest negative downturns in inorganic δ13C values from glacial initiation. These data also augment and temporally constrain the pre-glacial 87Sr/86Sr curve, informing ongoing modeling efforts of global weathering and seawater chemistry as they relate to hypotheses of the role of large igneous province emplacement and paleogeography in the initiation of Snowball Earth glaciation.

Detrital zircon U-Pb and (U-Th)/He double-dating of Upper Cretaceous-Cenozoic Zagros foreland basin strata in the Kurdistan Region of northern Iraq

NASA Astrophysics Data System (ADS)

Barber, D. E.; Stockli, D. F.; Koshnaw, R. I.; Horton, B. K.; Tamar-Agha, M. Y.; Kendall, J. J.

2014-12-01

The NW Zagros orogen is the result of the multistage collisional history associated with Late Cretaceous-Cenozoic convergence of the Arabian and Eurasian continents and final closure of Neotethys. Siliciclastic strata preserved within a ~400 km segment of the NW Zagros fold-thrust belt and foreland basin in the Iraqi Kurdistan Region (IKR) provide a widespread record of exhumation and sedimentation. As a means of assessing NW Zagros foreland basin evolution and chronostratigraphy, we present coupled detrital zircon (DZ) U-Pb and (U-Th)/He geo-thermochronometric data of Upper Cretaceous to Pliocene siliciclastic strata from the Duhok, Erbil, and Suleimaniyah provinces of IKR. LA-ICP-MS U-Pb age analyses reveal that the foreland basin fill in IKR in general was dominantly derived from Pan-African/Arabian-Nubian, Peri-Gondwandan, Eurasian, and Cretaceous volcanic arc terrenes. However, the provenance of these strata varies systematically along strike and through time, with an overall increase in complexity upsection. DZ age distribution of Paleocene-Eocene strata is dominated by a ~95 Ma grain age population, likely sourced from the Late Cretaceous Hassanbag-Bitlis volcanic arc complex along the northern margin of Arabia. In contrast, DZ U-Pb age distributions of Neogene strata show a major contribution derived from various Eurasian (e.g., Iranian, Tauride, Pontide; ~45, 150, 300 Ma) and Pan-African (~550, 950 Ma) sources. The introduction of Eurasian DZ ages at the Paleogene-Neogene transition likely records the onset of Arabian-Eurasian collision. Along strike to the southeast, the DZ U-Pb spectra of Neogene strata show a decreased percentage of Pan-African, Peri-Gondwandan, Tauride, and Ordovician ages, coupled with a dramatic increase in 40-50 Ma DZ ages that correspond to Urumieh-Dokhtar magmatic rocks in Iran. Combined with paleocurrent data, this suggests that Neogene sediments were transported longitudinally southeastward through an unbroken foreland basin system and progressively diluted downstream by detritus shed from the Iranian Plateau. Combined (U-Th)/He dating of DZ grains derived from the Hassanbag-Bitlis complex documents a major tectonothermal event at ~75 Ma, corresponding to the timing of proto-Zagros uplift and initial basin development in IKR.

Constraints on the timing of multiple thermal events and re-equilibration recorded by high-U zircon and xenotime: Case study of pegmatite from Piława Górna (Góry Sowie Block, SW Poland)

NASA Astrophysics Data System (ADS)

Budzyń, Bartosz; Sláma, Jiří; Kozub-Budzyń, Gabriela A.; Konečný, Patrik; Holický, Ivan; Rzepa, Grzegorz; Jastrzębski, Mirosław

2018-06-01

The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO2), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study demonstrates the importance of the careful examination of compositional, microtextural and geochronological data obtained using microanalytical techniques to reconstruct the complex thermal histories recorded by accessory minerals.

LA-ICP-MS Pb-U Dating of Young Zircons from the Kos-Nisyros Volcanic Centre, SE Aegean Arc (Greece)

NASA Astrophysics Data System (ADS)

Guillong, M.; Von Quadt, A.; Peytcheva, I.; Bachmann, O.

2014-12-01

Zircon Pb-U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos-Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in 230Th decay. Guillong, M. et al., 2014, JAAS, 29, p. 963-967; doi: 10.1039/c4ja00009a.

High precision time calibration of the Permian-Triassic boundary mass extinction event in a deep marine context

NASA Astrophysics Data System (ADS)

Baresel, Björn; Bucher, Hugo; Brosse, Morgane; Bagherpour, Borhan; Schaltegger, Urs

2015-04-01

To construct a revised and high resolution calibrated time scale for the Permian-Triassic boundary (PTB) we use (1) high-precision U-Pb zircon age determinations of a unique succession of volcanic ash layers interbedded with deep water fossiliferous sediments in the Nanpanjiang Basin (South China) combined with (2) accurate quantitative biochronology based on ammonoids, conodonts, radiolarians, and foraminifera and (3) tracers of marine bioproductivity (carbon isotopes) across the PTB. The unprecedented precision of the single grain chemical abrasion isotope-dilution thermal ionization mass spectrometry (CA-ID-TIMS) dating technique at sub-per mil level (radio-isotopic calibration of the PTB at the <100 ka level) now allows calibrating magmatic and biological timescales at resolution adequate for both groups of processes. Using these alignments allows (1) positioning the PTB in different depositional setting and (2) solving the age contradictions generated by the misleading use of the first occurrence (FO) of the conodont Hindeodus parvus, whose diachronous first occurrences are arbitrarily used for placing the base of the Triassic. This new age framework provides the basis for a combined calibration of chemostratigraphic records with high-resolution biochronozones of the Late Permian and Early Triassic. Here, we present new single grain U-Pb zircon data of volcanic ash layers from two deep marine sections (Dongpan and Penglaitan) revealing stratigraphic consistent dates over several volcanic ash layers bracketing the PTB. These analyses define weighted mean 206Pb/238U ages of 251.956±0.033 Ma (Dongpan) and 252.062±0.043 Ma (Penglaitan) for the last Permian ash bed. By calibration with detailed litho- and biostratigraphy new U-Pb ages of 251.953±0.038 Ma (Dongpan) and 251.907±0.033 Ma (Penglaitan) are established for the onset of the Triassic.

Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages

NASA Astrophysics Data System (ADS)

Chattopadhyay, Anupam; Chatterjee, Amitava; Das, Kaushik; Sarkar, Arindam

2017-10-01

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5 Ga) Betul Belt and the Neoproterozoic (∼1.0 Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300 km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MS U-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2 Ga and 0.95 Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0 Ga and 0.95 Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia.

Ore genesis and geodynamic setting of the Lianhuashan porphyry tungsten deposit, eastern Guangdong Province, SE China: constraints from muscovite 40Ar-39Ar and zircon U-Pb dating and Hf isotopes

NASA Astrophysics Data System (ADS)

Liu, Peng; Mao, Jingwen; Pirajno, Franco; Jia, Lihui; Zhang, Feng; Li, Yang

2017-12-01

The Lianhuashan deposit has long been regarded as a typical tungsten porphyry deposit, located in the eastern Guangdong Province, in the Southeastern Coastal Metallogenic Belt (SCMB). LA-MC-ICP-MS zircon U-Pb dating of the quartz porphyry yielded a weighted mean 206Pb/238U age of 137.3 ± 2.0 Ma, which is interpreted as the emplacement age of the quartz porphyry. Hydrothermal muscovite yielded a plateau 40Ar/39Ar age of 133.2 ± 0.9 Ma, which is consistent with the zircon U-Pb age, suggesting that the tungsten mineralization is genetically related to the quartz porphyry. Combined with previous studies, we suggest that there is a 145-135 Ma episode linking the granitic magmas with W-Sn ore systems in the SCMB. Zircon ɛHf (t) values of the quartz porphyry are in range of - 3.8 to 0.9, and the two-stage Hf model ages (TDM2) are 1.1-1.4 Ga, which is younger than the basement rocks in the Cathaysia Block (1.8-2.2 Ga), signifying that the quartz porphyry was predominantly derived from melting of Mesoproterozoic crust containing variable amounts of mantle components. In combination with the newly recognized coeval alkaline/bimodal magmatism and A-type granites in eastern Guangdong, we suggest that the 145-135 Ma W-Sn metallogenic event of the SCMB is related to a geodynamic setting of large-scale lithospheric extension and thinning, which can be ascribed to melting of the crust caused by mantle upwelling, triggered by the oblique subduction of the Izanagi plate.

Pb-Pb systematics of lunar rocks: differentiation, magmatic and impact history of the Moon

NASA Astrophysics Data System (ADS)

Nemchin, A.; Martin, W.; Norman, M. D.; Snape, J.; Bellucci, J. J.; Grange, M.

2016-12-01

Two independent decay chains in U-Pb system allow the determination of both ages and initial isotope compositions by analyzing only Pb in the samples. A typical Pb analysis represents a mixture of radiogenic Pb produced from the in situ U decay, initial Pb and laboratory contamination. Utilizing the ability of ion probes to analyse 10-30 micrometer-sized spots in the samples while avoiding fractures and other imperfections that commonly host contamination, permits extraction of pure lunar Pb compositions from the three component mixtures. This results in both accurate and precise ages of the rocks and their initial compositions. Lunar Mare and KREEP basalts postdating the major lunar bombardment are likely to represent such three component mixtures and are therefore appropriate for this approach, also giving an opportunity to investigate Pb evolution in their sources. A source evolution model constrained using available data indicates a major differentiation on the Moon at 4376±18 Ma and very radiogenic lunar mantle at this time. This age is likely to reflect the mean time of KREEP formation during the last stage of Magma Ocean differentiation. Rocks older than about 3.9 Ga are more complex than basalts and may include an extra Pb component, if modified by impacts. An example of this is presented by Pb-Pb data obtained for the anorthosite sample 62236, where the age of the rock is determined as 4367±29 Ma from analyses of CPx lamellae inside the large Opx grains: however large plagioclase crystals do not contain Pb in quantities sufficient for ion probe analysis, precluding determination of the initial Pb composition of the sample. Most of Pb is found in the brecciated parts of the anorthosite between the large grains. The composition of this Pb is similar to the initial Pb of 3909±17 Ma Apollo 16 breccia 66095, suggesting that is was injected into the anorthosite during a 3.9 Ga impact. Similar ca 3.9 Ga ages were determined for 1-2 millimeter size feldspathic clasts from several Apollo 14 breccias, where they are likely to date Pb homogenization during the Imbrium impact. Combined with U-Pb data obtained previously using U-bearing minerals such as zircon and phosphates, the new Pb-Pb data sets open an opportunity for a detailed chronological and isotopic investigation of lunar differentiation, magmatic evolution and impact history.

New high precision U-Pb calibration of the late Early-Triassic (Smithian-Spathian Boundary, South China)

NASA Astrophysics Data System (ADS)

Widmann, Philipp; Leu, Marc; Goudemand, Nicolas; Schaltegger, Urs; Bucher, Hugo

2017-04-01

Following the Permian-Triassic mass extinction (PTME), the Early Triassic is characterized by large short-lived perturbations of the global carbon cycle associated with radiation and extinction pulses of the biota. More stable conditions resumed in the Middle Triassic (Anisian). The exact ages and duration of these short-lived but intense radiation-extinction events as well as carbon cycle perturbations are poorly constrained and a robust intercalibration of U-Pb dates, biochronozones and carbon isotope fluctuations is still lacking. An accurate and precise time frame is essential in order to quantify the dynamics of the underlying mechanistic processes and to assess the validity of the various explanatory scenarios. The most drastic Early Triassic extinction occurred at the Smithian-Spathian boundary (SSB) and is associated with a globally recognized sharp positive excursion of the marine d13C signal. Based on the most recently published ages for the Permian-Triassic boundary (251.938 ± 0.029 Ma, Baresel et al., 2016) and for the Early-Middle Triassic boundary (247.05 ± 0.16 Ma, Ovtcharova et al., 2015), we know the Early Triassic lasted 4.9 myr. However, neither the position of the SSB nor the durations of the major biotic and abiotic events around the SSB are constrained by radiometric dates. Here, we will present new high precision, chemical abrasion, isotope dilution, thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages from single zircon crystals, sampled from closely spaced volcanic ash layers that bracket the SSB in the Nanpanjiang Basin (Guizhou province, South China). These ash layers are found in a mixed carbonate-siliciclastic, conodont-rich sedimentary succession (Luolou Formation) that is well calibrated biochronologically. We obtained best estimates of the ages of the SSB and associated events by applying Bayesian age modelling. References: Baresel, B., Bucher, H., Brosse, M., Cordey, F., Guodun, K., and Schaltegger, U., 2016. Precise age for the Permian-Triassic boundary in South China from high precision U-Pb geochronology and Bayesian age-depth modelling: Solid Earth Discussions, doi: 10 .5194 /se -2016 -145. Ovtcharova, M., Goudemand, N., Hammer, O., Guodun, K., Cordey, F., Galfetti, T., Schaltegger, U. and Bucher, H. 2015. Developing a strategy for accurate definition of a geological boundary through radio-isotopic and biochronological dating: the Early-Middle Triassic boundary (South China). Earth-Science Reviews, 146, 65-76.

Trace- and rare-earth element geochemistry and Pb-Pb dating of black shales and intercalated Ni-Mo-PGE-Au sulfide ores in Lower Cambrian strata, Yangtze Platform, South China

NASA Astrophysics Data System (ADS)

Jiang, Shao-Yong; Chen, Yong-Quan; Ling, Hong-Fei; Yang, Jing-Hong; Feng, Hong-Zhen; Ni, Pei

2006-08-01

The Lower Cambrian black shale sequence of the Niutitang Formation in the Yangtze Platform, South China, hosts an extreme metal-enriched sulfide ore bed that shows >10,000 times enrichment in Mo, Ni, Se, Re, Os, As, Hg, and Sb and >1,000 times enrichment in Ag, Au, Pt, and Pd, when compared to average upper continental crust. We report in this paper trace- and rare-earth-element concentrations and Pb-Pb isotope dating for the Ni-Mo-PGE-Au sulfide ores and their host black shales. Both the sulfide ores and their host black shales show similar trace-element distribution patterns with pronounced depletion in Th, Nb, Hf, Zr, and Ti, and extreme enrichment in U, Ni, Mo, and V compared to average upper crust. The high-field-strength elements, such as Zr, Hf, Nb, Ta, Sc, Th, rare-earth elements, Rb, and Ga, show significant inter-element correlations and may have been derived mainly from terrigenous sources. The redox sensitive elements, such as V, Ni, Mo, U, and Mn; base metals, such as Cu, Zn, and Pb; and Sr and Ba may have been derived from mixing of seawater and venting hydrothermal sources. The chondrite-normalized REE patterns, positive Eu and Y anomalies, and high Y/Ho ratios for the Ni-Mo-PGE-Au sulfide ores are also suggestive for their submarine hydrothermal-exhalative origin. A stepwise acid-leaching Pb-Pb isotope analytical technique has been employed for the Niutitang black shales and the Ni-Mo-PGE-Au sulfide ores, and two Pb-Pb isochron ages have been obtained for the black shales (531±24 Ma) and for the Ni-Mo-PGE-Au sulfide ores (521±54 Ma), respectively, which are identical and overlap within uncertainty, and are in good agreement with previously obtained ages for presumed age-equivalent strata.

Newly Described Tephra Provide Middle Pleistocene Age Constraints to Stegodon Fossils in West (Indonesian) Timor

NASA Astrophysics Data System (ADS)

Jensen, B. J. L.; Dufrane, A.; Mark, D.; Zaim, Y.; Rizal, Y.; Aswan, A.; Hascaryo, A.; Ciochon, R.; Gunnell, G.; Larick, R.; Zonnveld, J. P.

2017-12-01

As the Asian proboscidian Stegodon dispersed across Island Southeast Asia during the Pleistocene, multiple forms developed. On Timor, a southerly island east of Wallace's Line, the Ainaro gravels have yielded a highly dwarfed S. timorensis and a larger S. `trigonocephalus.' During a half-century of exploration, the age of the fossil bearing gravels remains in question, with only one age determination of >130 ka derived from six 230Th- 238U dates on a tusk fragment found in the Raebia area (Louys et al. 2016). Here we present radiometric ages for two tephra deposits bracketing Ainaro gravels at Raebia, a S. timorensis fossil locality 8 km northeast of Atambua city. The Raebia ravine exposes 2-10 meters of coarse-grained gravels incised into silt and clay deposits, bracketed by two indurated and largely devitrified tephras. Some intact glass was present to geochemically characterize each unit, which are both high-silica rhyolites. Biotite and zircons for 40Ar/39Ar and laser ablation U-Pb dating were extracted from the upper unit (Raebia Tuff 1; RT1), and zircons from the lower unit (Raebia Tuff 2; RT2). RT1 had zircons with two distinct age populations, but the youngest yield a 230Th deficiency corrected 206Pb/238U age of 665 ± 19 ka, (2s, n = 23, MSWD = 0.81), consistent with the 40Ar/39Ar age 614.9 ± 16.4 ka (2s, full external precision). Preliminary zircon dates on RT2 are more problematic, providing a large range that suggests inheritance by xenoliths and/or locally-sourced detrital zircons. However, a single zircon yielded 230Th deficiency corrected 206Pb/238U age of 708 ± 66 ka (2s, n=17, MSWD = 0.41), which is stratigraphically consistent. These are the first reliable age constraints on a higher elevation Ainaro gravel terrace and fossils they contain. The only other direct ages on the gravels are 230Th- 238U dates on lower terraces interbedded with coral, ranging from 130 ka to Holocene in age (Roosmawati and Harris 2009). These two newly described and dated tephra are likely regionally distributed and may represent important stratigraphic horizons for this portion of Southern Wallacea. They also provide useful data for calculating uplift rates for the region from the middle Pleistocene.

Dating High Temperature Mineral Fabrics in Lower Crustal Granulite Facies Rocks

NASA Astrophysics Data System (ADS)

Stowell, H. H.; Schwartz, J. J.; Tulloch, A. J.; Klepeis, K. A.; Odom Parker, K.; Palin, M.; Ramezani, J.

2015-12-01

Granulite facies rocks may record strain that provides a record of compressional and/or extensional crustal events in hot orogenic cores and the roots of magmatic arcs. Although the precise timing of these events is important for constructing tectonic histories, it is often difficult to determine due to uncertain relationships between isotopic signatures, mineral growth, and textural features that record strain. In addition, there may be large uncertainties in isotope data due to intracrystalline diffusion and multiple crystallization events. L-S tectonites in lower crustal rocks from Fiordland, NZ record the early stages of extensional collapse of thickened magmatic arc crust. The precise age of these fabrics is important for constraining the timing of extension that led to opening of the Tasman Sea. High temperature granulite facies L-S fabrics in garnet reaction zones (GRZ) border syn- to post-deformational leucosomes. U-Pb zircon, Lu-Hf garnet, and Sm-Nd garnet ages, and trace elements in these phases indicate the complexity of assigning precise and useful ages. Zircon have soccer ball morphology with patchy and sector zoned CL. Zircon dates for igneous host and adjacent GRZ range over ca. 17 Ma. 236U-208Pb LA-ICP-MS are 108-125 Ma, N=124 (host & GRZ); however, chemical abrasion (CA) shifts GRZ dates ca. 2 Ma older. 236U-208Pb SHRIMP-RG dates cluster in 2 groups: 118.5±0.8 Ma, N=23 and 111.0±0.8 Ma, N=6. CA single crystal TIMS dates also fall into 2 groups: 117.6±0.1 Ma, N=4 and 116.6±0.2 Ma N=4. Garnet isochron ages determined from coarse garnet selvages adjacent to leucosomes range from 112.8±2.2 (147Sm-143Nd, 10 pts.) to 114.8±3.5 (177Lu-176Hf, 6 pts.) Ma. Zircon dates from all methods show ranges (>10 Ma) and 2 distinct populations. Host and GRZ zircon cannot be readily distinguished by age, lack younger rims, but have distinct Th/U trends and Eu/Eu* vs. Hf ratios. Difference in zircon trace element composition indicates either early leucosome emplacement or xenocrystic zircon in leucosomes. We conclude that the small number of oldest zircon grains are inherited, older zircon age populations (CA LA-ICP-MS, SHRIMP-RG and TIMS) are near identical ca. 118 Ma and date intrusion, and that the youngest zircon and indistinguishable garnet ages (113-116 Ma) date syn-deformational granulite facies metamorphism.

Elemental and Molecular Segregation in Oil Paintings due to Lead Soap Degradation.

PubMed

Chen-Wiegart, Yu-Chen Karen; Catalano, Jaclyn; Williams, Garth J; Murphy, Anna; Yao, Yao; Zumbulyadis, Nicholas; Centeno, Silvia A; Dybowski, Cecil; Thieme, Juergen

2017-09-14

The formation of Pb, Zn, and Cu carboxylates (soaps) has caused visible deterioration in hundreds of oil paintings dating from the 15th century to the present. Through transport phenomena not yet understood, free fatty acids in the oil binding medium migrate through the paint and react with pigments containing heavy metals to form soaps. To investigate the complex correlation among the elemental segregation, types of chemical compounds formed, and possible mechanisms of the reactions, a paint sample cross-section from a 15th century oil painting was examined by synchrotron X-ray techniques. X-ray fluorescence (XRF) microscopy, quantified with elemental correlation density distribution, showed Pb and Sn segregation in the soap-affected areas. X-ray absorption near edge structure (XANES) around the Pb-L3 absorption edge showed that Pb pigments and Pb soaps can be distinguished while micro-XANES gave further information on the chemical heterogeneity in the paint film. The advantages and limitations of these synchrotron-based techniques are discussed and compared to those of methods routinely used to analyze paint samples. The results presented set the stage for improving the information extracted from samples removed from works of art and for correlating observations in model paint samples to those in the naturally aged samples, to shed light onto the mechanism of soap formation.

Development of recent chronologies and evaluation of temporal variations in Pb fluxes and sources in lake sediment and peat cores in a remote, highly radiogenic environment, Cairngorm Mountains, Scottish Highlands

NASA Astrophysics Data System (ADS)

Farmer, John G.; MacKenzie, Angus B.; Graham, Margaret C.; Macgregor, Kenneth; Kirika, Alexander

2015-05-01

The use of stable Pb isotope analyses in conjunction with recent (210Pb and anthropogenic radionuclide) chronologies has become a well-established method for evaluating historical trends in depositional fluxes and sources of atmospherically deposited Pb using archival records in lake sediment or peat cores. Such studies rely upon (i) simple radioactive disequilibrium between unsupported 210Pb and longer-lived members of the 238U decay series and (ii) well-defined values for the isotopic composition of contaminant Pb and indigenous Pb in the study area. However, areas of high natural radioactivity can present challenging environments for such studies, with potential complications arising from more complex disequilibria in the 238U decay series and the occurrence, at local or regional level, of anomalous, ill-defined stable isotope ratios due to the presence of elevated levels of radiogenic Pb. Results are presented here for a study of a sediment core from a freshwater lake, Loch Einich, in the high natural radioactivity area of the Cairngorm Mountains of Scotland. 238U decay series disequilibria revealed recent diagenetic re-deposition of both U and 226Ra, the latter resulting in a requirement to use a modified calculation to derive a 210Pb chronology for the core. Confidence in the chronology was provided by good agreement with the independent 241Am chronology, but the 137Cs distribution was affected by significant post-depositional mobility in the organic-rich sediment. The systematics of variations in 230Th, 232Th and stable Pb isotope ratio distributions were used to establish the indigenous Pb characteristics of the sediment. The relatively high radiogenic content of the indigenous Pb resulted in complications in source apportionment, in particular during the 20th century, with multiple natural and anthropogenic sources precluding the use of a simple binary mixing model. Consequently, 206Pb/207Pb ratios in Scottish moss samples from an archive collection were used to provide the input term for atmospheric deposition in order to establish historical trends in indigenous and anthropogenic Pb fluxes. A test of the accuracy of the derived Pb fluxes was provided by analysis of a core from a nearby blanket peat deposit, Great Moss. Independent atmospheric and basal inputs gave a complex distribution of 210Pb in the peat, but this did not affect calculation of a 210Pb chronology. Once again, the 210Pb chronology was supported by the 241Am distribution. Temporal trends in anthropogenic Pb deposition derived for the Loch Einich sediment core were in generally good agreement with those for the Great Moss peat core, other peat cores and some other lake sediment cores from northern Scotland, providing confidence in the use of the archive moss data to characterise atmospheric deposition. However, sustained input of Pb to Loch Einich sediment at relatively high levels in the late 20th century, after the regional decline in atmospheric Pb deposition, suggested that catchment-derived Pb is now a significant component of the depositional flux for Loch Einich.

Evidence for large-scale submarine mass wasting associated with growth of oceanic core complexes, 16°N, Mid-Atlantic Ridge.

NASA Astrophysics Data System (ADS)

Urann, B.; Cheadle, M. J.; John, B. E.; Dick, H. J.

2016-12-01

Slow spreading ridges display distinct geomorphologic features, often interpreted as long-lived detachment faults, where mafic and ultramafic rocks are exposed at the seafloor. Many bathymetric features in these regions are viewed as the result of tectonic processes (long lived detachment faults), however other features are clearly the result of mass wasting. Here we report zircon U-Pb dates from four gabbro and Fe-Ti oxide gabbro dredge samples recovered from the North Segment on the western flank of the mid-Atlantic ridge (MAR) at 16°N. Initial SIMS U-Pb zircon dating using the Stanford-USGS SHRIMP-RG ion-microprobe yield 230 Th-corrected zircon 206Pb/238U dates within error of one another. The two westernmost samples are separated by 14km along strike, and come from dredges on the footwall of a high-angle normal fault. They have dates of 1.112 +/-0.083 Ma and 1.181 +/- 0.074Ma, and both lie 12-13km west of the present day axial volcanic ridge, These samples therefore yield a spreading rate of 12km/Ma, as expected for this part of the MAR. The two eastern samples lie up to 4.5 km east of the western samples and yield dates of 1.14_/-0.55Ma and 1,221+/-0.027Ma, indistinguishable from those of the samples to the west. Given the predicted spreading rate of 12 km/Ma, these samples should be 0.375Ma younger than those to the west, and should yield dates of 0.74Ma. To account for the similarity in age, we suggest that mass wasting and large landslides from the high angle fault scarps displaced as much as 40km3 of material into the axial valley, dispersing gabbro of similar age over a wide area. This interpretation is consistent with the available multi-beam bathymetry that can be explained in terms of large landslides flowing from the bounding fault scarps into the axial valley towards the present day axial volcanic ridge. If correct, this interpretation has significant implications for evaluating potential tsunami hazards at mid-ocean ridges.

The Diamondiferous Lithospheric Mantle Underlying the Eastern Superior Craton: Evidence From Mantle Xenoliths From the Renard Kimberlites, Quebec

NASA Astrophysics Data System (ADS)

Hunt, L.; Stachel, T.; Armstrong, J. P.; Simonetti, A.

2009-05-01

The Renard kimberlite cluster consists of nine pipes located within a 2km2 area in the northern Otish Mountains of Quebec. The pipes are named Renards 1 to 10, with subsequent investigation revealing Renards 5 and 6 to join at depth (now Renard 65). The pipes are located within the eastern portion of the Superior craton, emplaced into Archean granitic and gneissic host rocks of the Opinica Subprovince (Percival, 2007). Amphibolite grade metamorphism, locally passing into the granulite facies (Percival et al., 1994) occurred in late Archean time (Moorhead et al., 2003). Radiometric dating of the hypabyssal Renard 1 kimberlite indicates Neoproterozoic emplacement, with a 206Pb/238U model age of 631.6±3.5 Ma (2σ) (Birkett et al., 2004). A later study on the main phases in Renard 2 and 3 gave a similar emplacement, with a 206Pb/238U model age of 640.5±2.8Ma (Fitzgerald et al., 2008). This makes this kimberlite district one of the oldest in Canada, similar in eruption age to the Wemindji kimberlites (629±29Ma: Letendre et al., 2003). These events are broadly coeval with the conversion from subduction magmatism to rifting in northern Laurentia (Birkett et al., 2004). The bodies are part of a late Neoproterozoic to Cambrian kimberlite field in eastern Canada (Girard, 2001; Moorhead et al, 2002; Letendre et al., 2003) and fit into the north-east of the Eocambrian/Cambrian Labrador Sea Province of Heaman et al. (2004). To better understand the diamondiferous lithospheric mantle beneath the Renard kimberlites, 116 microxenoliths and xenocrysts were analysed. The samples were dominantly peridotitic, composed primarily of purple garnet, emerald green clinopyroxene and olivine, with a few pink and red garnets. A minor eclogitic component comprises predominantly orange garnets and lesser amounts of clinopyroxene. A detailed study on the major, minor and trace element composition of xenolith minerals is currently underway. All but three of the clinopyroxenes analysed to date plot into the on-craton garnet peridotite field of Ramsay (1992), and follow the garnet peridotite trend of Grütter (2008). Using the single pyroxene geothermobarometer of Nimis and Taylor (2000), the clinopyroxene grains fall along a 38mW/m2 model geotherm. However, the majority fall on the low pressure side of the diamond graphite transition. Initial analysis on the garnet grains show that the majority plots in the on craton lherzolite field (G9A) of Grütter et al. (2006). A smaller eclogite population is also present, along with a minor harzburgitic (G10) population. Using the manganese in garnet thermometer of Creighton (2008) the majority of grains fall in the diamond window (T>950°C). This indicates a currently unexplained disconnect between clinopyroxene and garnet geothermobarometry. The newly developed technique of in situ Pb-Pb dating of clinopyroxene xenocrysts (Schmidberger et al. 2007) was applied to the microxenoliths. Initial results indicate an age of ˜2.7 Ga for the subcratonic lithospheric mantle beneath Renard. This date is significant, coinciding with the beginning of the break up of Vaalbara and a major phase of continental crust generation. Also at 2.7 Ga, Kenorland (including the Superior Province) was formed by accretion of granitoid-greenstone terranes at convergent margins (Barley et al., 2005).

U-Pb dating of uranium deposits in collapse breccia pipes of the Grand Canyon region

USGS Publications Warehouse

Ludwig, K. R.; Simmons, K.R.

1992-01-01

Two major periods of uranium mineralization are indicated by U-Pb isotope dating of uranium ores from collapse breccia pipes in the Grand Canyon region, northern Arizona. The Hack 2 and 3, Kanab North, and EZ 1 and 2 orebodies apparently formed in the interval of 200 ?? 20 Ma, similar to ages inferred for strata-bound, Late Triassic-hosted uranium deposits in southern Utah and northern Arizona. Samples from the Grand Canyon and Pine Nut pipes, however, indicate a distinctly older age of about 260 Ma. The clustering in ages for a variety of uranium deposits at about the age of the lower part of the Chinle Formation (Late Triassic) suggests that uranium in these deposits may have been derived by leaching from volcanic ash in the Chinle and mobilized by ground-water movement. Pb isotope ratios of galenas in mineralized pipes are more radiogenic than those of sulfides from either uranium-poor pipes or occurrences away from pipes. Fluids which passed through the pipes had interacted with the Proterozoic basement, possibly through the vertical fractures which influenced the location and evolution of the pipes themselves. -from Authors

Developing an Age Model for the Cryogenian: the South China perspective

NASA Astrophysics Data System (ADS)

Condon, D. J.; Zhu, M.; Macdonald, F. A.; Tapster, S.

2014-12-01

The Neoproterozoic stratigraphic record of South China contains distributed volcanic ash beds, which can be dated and used to provide a backbone for Cryogenian and Ediacaran 'age models'. When integrated with geochemical and paleontological data, both locally and from other regions, these successions inform our understanding of environmental and biological change from these intervals. At present the South China stratigraphy provides the dates that best approximate the termination of the Marinoan glaciation (ca. 635 Ma, U-Pb single zircon isotope dilution1) from a dated volcanic ash bed that occurs in the cap carbonate sequence (Doushantuo Formation) which directly overlies the Nantuo Tillite (Marinoan glaciation equivalent). A number of other ash beds have been documented in both the Nantuo Tillite and throughout the Datangpo Formation (pre-Marinoan non-glacial succession) and have been variably dated using U-Pb microbeam and multi-grain isotope dilution studies2-4. These published data sets have produced dates that have uncertainties on the order of 4 Myr but are built upon single-analyses data points, which have uncertainties on the order of 10 Myr or greater thereby limiting their utility for assessing temporal relationship with other dated section. We present new U-Pb single zircon isotope dilution geochronology data from a suite of samples from the Datangpo, Nantuo and Doushantuo Formations. New dates from the base of the Datangpo Formation confirm a ca. 660 Ma end to the Sturtian glacial epoch, which is represented by underlying glacial deposits. Additional dates from ash beds from the Datangpo Formation and the lower Nantuo Tillite will refine the duration of the Cryogenian non-glacial interlude and Marinoan glaciation as recorded in South China. Combined with published data from Namibia, Australia and Canada, these new data will permit the development of a more highly resolved age model for the Cryogenian, enabling estimation of duration and synchronicity of proxy records. They will also provide further tests and input data for numerical models that seek to simulate Cryogenian climate drivers and responses. 1. D. Condon et al., (2005) Science308, 95; 2. C. M. Zhou et al., (2004) Geology32, 437; 3. S. Zhang, et al., (2008) Terra Nova20, 289; 4. S. Zhang et al., (2005) Geology 33, 473

U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa.

PubMed

Mungall, James E; Kamo, Sandra L; McQuade, Stewart

2016-11-14

Layered intrusions represent part of the plumbing systems that deliver vast quantities of magma through the Earth's crust during the formation of large igneous provinces, which disrupt global ecosystems and host most of the Earth's endowment of Pt, Ni and Cr deposits. The Rustenburg Layered Suite of the enormous Bushveld Igneous Complex of South Africa has been presumed to have formed by deposition of crystals at the floor of a subterranean sea of magma several km deep and hundreds of km wide called a magma chamber. Here we show, using U-Pb isotopic dating of zircon and baddeleyite, that individual chromitite layers of the Rustenburg Layered Suite formed within a stack of discrete sheet-like intrusions emplaced and solidified as separate bodies beneath older layers. Our U-Pb ages and modelling necessitate reassessment of the genesis of layered intrusions and their ore deposits, and challenge even the venerable concept of the magma chamber itself.

Unraveling the Switch from Subduction to Exhumation within a Collisional Orogen: Split-stream U-Pb and Trace-element Results from the Western Gneiss Region, Norway (Invited)

NASA Astrophysics Data System (ADS)

Gordon, S. M.; Whitney, D. L.; Teyssier, C. P.; Fossen, H.; Desormeau, J. W.; Jessen, B.

2013-12-01

During continental collision, crustal material may be subducted to great depths and subsequently exhumed. Ultrahigh-pressure (UHP) terranes preserve a record of the subduction of crustal material during suturing of colliding continents and the exhumation of this material during extension and, in some cases, collapse of the orogen. The UHP rocks of the Western Gneiss Region (WGR), Norway, resulted from the collision of Baltica with Laurentia during the final stages of the Caledonian orogeny. The WGR represents one of the two largest UHP terranes on Earth and consists of a UHP eclogite-bearing domain south of the Møre-Trøndelag strike-slip fault and a HP mafic granulite-bearing domain north of the fault. At least some of the HP granulite is overprinted eclogite. To evaluate the metamorphic and structural relationship of mafic rocks and associated migmatite in both regions, we obtained LA-ICP-MS U-Pb dates and trace-element analyses for zircon from a variety of textural types of leucosome associated with mafic layers and lenses. Five leucosomes within highly deformed migmatite in the HP granulite complex on the Roan Peninsula reveal U-Pb lower-intercept ages from ca. 405 to 409 Ma and upper-intercept Proterozoic dates. These zircons have distinct trace-elements patterns: all of the zircons that yield Proterozoic dates have overall much higher REE concentrations, a more significant negative Eu anomaly (-0.3 to -0.7) and steeper HREE patterns (Lu/Dy = 5-12). In comparison, the Caledonian zircons reveal flatter Eu anomalies (-0.3 to 0.2) and less steep HREE patterns (Lu/Dy = 2-7), although the individual patterns do not seem to correlate with age. The Caledonian zircon patterns suggest crystallization at high-pressures and are distinct from the inherited Proterozoic grains. Similar results were obtained from zircon rims extracted from layer-parallel to crosscutting leucosomes from the UHP domain. Trace elements in zircon in these samples record the transition from high-pressure (garnet-present, plagioclase-absent) crystallization to lower-pressure (plagioclase-present) crystallization with garnet-present × plagioclase-absent REE patterns. Moreover, dates from the layer-parallel leucosomes are as old as 410-406 Ma. The new U-Pb dates suggest a similar melt crystallization history that was coeval with previously determined ages of (U)HP metamorphism of WGR eclogite. The More-Trondelag fault acted as a transform fault and accommodated coeval extension that exhumed both the (U)HP and granulite domains. Results are consistent with the presence of partially molten crust in a large part of the WGR at HP or UHP conditions during the latest stages of the Caledonian orogeny. The decreased viscosity and increased buoyancy and strain weakening induced by partial melting may have triggered or at least contributed to the switch from subduction to exhumation in the WGR, marking the end of collisional orogeny.

Time-scale calibration by U-Pb geochronology: Examples from the Triassic Period

NASA Astrophysics Data System (ADS)

Mundil, R.

2009-05-01

U-Pb zircon geochronology, pioneered by Tom Krogh, is a cornerstone for the calibration of the time scale. Before Krogh's innovations, U-Pb geochronology was essentially limited by laboratory blank Pb (typically hundreds of nanograms) inherent in the then existing zircon dissolution and purification methods. The introduction of high pressure HF dissolution combined with miniature ion exchange columns (1) reduced the blank by orders of magnitude and allowed mass-spectrometric analyses of minute amounts of material (picograms of Pb and U). Krogh also recognized the need for minimizing the effects of Pb loss, and the introduction of the air-abrasion technique was the method of choice for two decades (2), until the development of the combined annealing and chemical abrasion technique resulted in essentially closed system zircons (3). These are the prerequisite for obtaining precise (permil-level) and accurate radio-isotopic ages of individual zircons contained in primary volcanic ash deposits, which are primary targets for the calibration of the time scale if they occur within fossil bearing sediments. A prime example is the calibration of the Triassic time scale which improved significantly using these techniques. The ages for the base and the top of the Triassic are constrained by U-Pb ages to 252.3 (4) and 201.5 Ma (5), respectively. These dates also constrain the ages of major extinction events at the Permian-Triassic and Triassic-Jurassic boundaries, and are statistically indistinguishable from ages obtained for the Siberian Traps and volcanic products from the Central Atlantic Magmatic Province, respectively, suggesting a causal link. Ages for these continental volcanics, however, are mostly from the K-Ar (40Ar/39Ar) system which requires accounting and correcting for a systematic bias of ca 1 % between U-Pb and 40Ar/39Ar isotopic ages (the 40Ar/39Ar ages being younger) (6). Robust U-Pb age constraints also exist for the Induan- Olenekian boundary (251.2 Ma, (7)) and the Early-Middle Triassic (Olenekian-Anisian) boundary (247.2 Ma, (8, 9)), resulting in a surprisingly short duration of the Early Triassic which has implications for the timing of biotic recovery and major changes in ocean chemistry during this time. Furthermore, the Anisian-Ladinian boundary is constrained to 242.0 Ma by new U-Pb and 40Ar/39Ar ages. Radio-isotopic ages for the Late Triassic are scarce and the only reliable and biostratigraphically controlled age is from an upper Carnian tuff dated to 230.9 Ma (10), yielding a duration of more than 35 Ma for the Late Triassic. The resulting time-scale is at odds with the most recent compilation (11) but arguably more accurate because it is entirely based on U-Pb analyses applied to closed-system zircons with uncertainties at the permil level or better. 1. T. E. Krogh, Geochimica et Cosmochimica Acta 37, 485 (1973); 2. T. E. Krogh, Geochimica et Cosmochimica Acta 46, 637 (1982); 3. J. M. Mattinson, Chemical Geology 220, 47 (2005); 4. R. Mundil, K. R. Ludwig, I. Metcalfe, P. R. Renne, Science 305, 1760 (2004); 5. U. Schaltegger, J. Guex, A. Bartolini, B. Schoene, M. Ovtcharova, Earth and Planetary Science Letters 267, 266 (2008); 6. R. Mundil, P. R. Renne, K. K. Min, K. R. Ludwig, in Eos Trans. AGU, Fall Meet. Suppl. (2006), vol. 87(52), pp. V21A-0543; 7. T. Galfetti et al., Earth and Planetary Science Letters 258, 593 (2007). 8. M. Ovtcharova et al., Earth and Planetary Science Letters 243, 463 (2006). 9. J. Ramezani et al., Earth and Planetary Science Letters 256, 244 (2007). 10. S. Furin et al., Geology 34, 1009 (2006); 11. J. G. Ogg, in A Geologic Time Scale 2004 F. M. Gradstein, J. G. Ogg, A. G. Smith, Eds. (University Press, Cambridge, 2004) pp. 271-306.

Search for diphoton events with large missing transverse energy with 36 pb-1 of 7 TeV proton-proton collision data with the ATLAS detector

NASA Astrophysics Data System (ADS)

Aad, G.; Abbott, B.; Abdallah, J.; Abdelalim, A. A.; Abdesselam, A.; Abdinov, O.; Abi, B.; Abolins, M.; Abramowicz, H.; Abreu, H.; Acerbi, E.; Acharya, B. S.; Adams, D. L.; Addy, T. N.; Adelman, J.; Aderholz, M.; Adomeit, S.; Adragna, P.; Adye, T.; Aefsky, S.; Aguilar-Saavedra, J. A.; Aharrouche, M.; Ahlen, S. P.; Ahles, F.; Ahmad, A.; Ahsan, M.; Aielli, G.; Akdogan, T.; Åkesson, T. P. A.; Akimoto, G.; Akimov, A. V.; Akiyama, A.; Alam, M. S.; Alam, M. A.; Albert, J.; Albrand, S.; Aleksa, M.; Aleksandrov, I. N.; Alessandria, F.; Alexa, C.; Alexander, G.; Alexandre, G.; Alexopoulos, T.; Alhroob, M.; Aliev, M.; Alimonti, G.; Alison, J.; Aliyev, M.; Allport, P. P.; Allwood-Spiers, S. E.; Almond, J.; Aloisio, A.; Alon, R.; Alonso, A.; Alviggi, M. G.; Amako, K.; Amaral, P.; Amelung, C.; Ammosov, V. V.; Amorim, A.; Amorós, G.; Amram, N.; Anastopoulos, C.; Andari, N.; Andeen, T.; Anders, C. F.; Anderson, K. J.; Andreazza, A.; Andrei, V.; Andrieux, M.-L.; Anduaga, X. S.; Angerami, A.; Anghinolfi, F.; Anjos, N.; Annovi, A.; Antonaki, A.; Antonelli, M.; Antonov, A.; Antos, J.; Anulli, F.; Aoun, S.; Aperio Bella, L.; Apolle, R.; Arabidze, G.; Aracena, I.; Arai, Y.; Arce, A. T. H.; Archambault, J. P.; Arfaoui, S.; Arguin, J.-F.; Arik, E.; Arik, M.; Armbruster, A. J.; Arnaez, O.; Arnault, C.; Artamonov, A.; Artoni, G.; Arutinov, D.; Asai, S.; Asfandiyarov, R.; Ask, S.; Åsman, B.; Asquith, L.; Assamagan, K.; Astbury, A.; Astvatsatourov, A.; Atoian, G.; Aubert, B.; Auerbach, B.; Auge, E.; Augsten, K.; Aurousseau, M.; Austin, N.; Avramidou, R.; Axen, D.; Ay, C.; Azuelos, G.; Azuma, Y.; Baak, M. A.; Baccaglioni, G.; Bacci, C.; Bach, A. M.; Bachacou, H.; Bachas, K.; Bachy, G.; Backes, M.; Backhaus, M.; Badescu, E.; Bagnaia, P.; Bahinipati, S.; Bai, Y.; Bailey, D. C.; Bain, T.; Baines, J. T.; Baker, O. K.; Baker, M. D.; Baker, S.; Dos Santos Pedrosa, F. Baltasar; Banas, E.; Banerjee, P.; Banerjee, Sw.; Banfi, D.; Bangert, A.; Bansal, V.; Bansil, H. S.; Barak, L.; Baranov, S. P.; Barashkou, A.; Barbaro Galtieri, A.; Barber, T.; Barberio, E. L.; Barberis, D.; Barbero, M.; Bardin, D. Y.; Barillari, T.; Barisonzi, M.; Barklow, T.; Barlow, N.; Barnett, B. M.; Barnett, R. M.; Baroncelli, A.; Barone, G.; Barr, A. J.; Barreiro, F.; Barreiro Guimarães da Costa, J.; Barrillon, P.; Bartoldus, R.; Barton, A. E.; Bartsch, D.; Bartsch, V.; Bates, R. L.; Batkova, L.; Batley, J. R.; Battaglia, A.; Battistin, M.; Battistoni, G.; Bauer, F.; Bawa, H. S.; Beare, B.; Beau, T.; Beauchemin, P. H.; Beccherle, R.; Bechtle, P.; Beck, H. P.; Beckingham, M.; Becks, K. H.; Beddall, A. J.; Beddall, A.; Bedikian, S.; Bednyakov, V. A.; Bee, C. P.; Begel, M.; Harpaz, S. Behar; Behera, P. K.; Beimforde, M.; Belanger-Champagne, C.; Bell, P. J.; Bell, W. H.; Bella, G.; Bellagamba, L.; Bellina, F.; Bellomo, M.; Belloni, A.; Beloborodova, O.; Belotskiy, K.; Beltramello, O.; Ami, S. Ben; Benary, O.; Benchekroun, D.; Benchouk, C.; Bendel, M.; Benedict, B. H.; Benekos, N.; Benhammou, Y.; Benjamin, D. P.; Benoit, M.; Bensinger, J. R.; Benslama, K.; Bentvelsen, S.; Berge, D.; Bergeaas Kuutmann, E.; Berger, N.; Berghaus, F.; Berglund, E.; Beringer, J.; Bernardet, K.; Bernat, P.; Bernhard, R.; Bernius, C.; Berry, T.; Bertin, A.; Bertinelli, F.; Bertolucci, F.; Besana, M. I.; Besson, N.; Bethke, S.; Bhimji, W.; Bianchi, R. M.; Bianco, M.; Biebel, O.; Bieniek, S. P.; Biesiada, J.; Biglietti, M.; Bilokon, H.; Bindi, M.; Binet, S.; Bingul, A.; Bini, C.; Biscarat, C.; Bitenc, U.; Black, K. M.; Blair, R. E.; Blanchard, J.-B.; Blanchot, G.; Blazek, T.; Blocker, C.; Blocki, J.; Blondel, A.; Blum, W.; Blumenschein, U.; Bobbink, G. J.; Bobrovnikov, V. B.; Bocchetta, S. S.; Bocci, A.; Boddy, C. R.; Boehler, M.; Boek, J.; Boelaert, N.; Böser, S.; Bogaerts, J. A.; Bogdanchikov, A.; Bogouch, A.; Bohm, C.; Boisvert, V.; Bold, T.; Boldea, V.; Bolnet, N. M.; Bona, M.; Bondarenko, V. G.; Boonekamp, M.; Boorman, G.; Booth, C. N.; Bordoni, S.; Borer, C.; Borisov, A.; Borissov, G.; Borjanovic, I.; Borroni, S.; Bos, K.; Boscherini, D.; Bosman, M.; Boterenbrood, H.; Botterill, D.; Bouchami, J.; Boudreau, J.; Bouhova-Thacker, E. V.; Boulahouache, C.; Bourdarios, C.; Bousson, N.; Boveia, A.; Boyd, J.; Boyko, I. R.; Bozhko, N. I.; Bozovic-Jelisavcic, I.; Bracinik, J.; Braem, A.; Branchini, P.; Brandenburg, G. W.; Brandt, A.; Brandt, G.; Brandt, O.; Bratzler, U.; Brau, B.; Brau, J. E.; Braun, H. M.; Brelier, B.; Bremer, J.; Brenner, R.; Bressler, S.; Breton, D.; Britton, D.; Brochu, F. M.; Brock, I.; Brock, R.; Brodbeck, T. J.; Brodet, E.; Broggi, F.; Bromberg, C.; Brooijmans, G.; Brooks, W. K.; Brown, G.; Brown, H.; Bruckman de Renstrom, P. A.; Bruncko, D.; Bruneliere, R.; Brunet, S.; Bruni, A.; Bruni, G.; Bruschi, M.; Buanes, T.; Bucci, F.; Buchanan, J.; Buchanan, N. J.; Buchholz, P.; Buckingham, R. M.; Buckley, A. G.; Buda, S. I.; Budagov, I. A.; Budick, B.; Büscher, V.; Bugge, L.; Buira-Clark, D.; Bulekov, O.; Bunse, M.; Buran, T.; Burckhart, H.; Burdin, S.; Burgess, T.; Burke, S.; Busato, E.; Bussey, P.; Buszello, C. P.; Butin, F.; Butler, B.; Butler, J. M.; Buttar, C. M.; Butterworth, J. M.; Buttinger, W.; Byatt, T.; Cabrera Urbán, S.; Caforio, D.; Cakir, O.; Calafiura, P.; Calderini, G.; Calfayan, P.; Calkins, R.; Caloba, L. P.; Caloi, R.; Calvet, D.; Calvet, S.; Camacho Toro, R.; Camarri, P.; Cambiaghi, M.; Cameron, D.; Campana, S.; Campanelli, M.; Canale, V.; Canelli, F.; Canepa, A.; Cantero, J.; Capasso, L.; Capeans Garrido, M. D. M.; Caprini, I.; Caprini, M.; Capriotti, D.; Capua, M.; Caputo, R.; Caramarcu, C.; Cardarelli, R.; Carli, T.; Carlino, G.; Carminati, L.; Caron, B.; Caron, S.; Carrillo Montoya, G. D.; Carter, A. A.; Carter, J. R.; Carvalho, J.; Casadei, D.; Casado, M. P.; Cascella, M.; Caso, C.; Castaneda Hernandez, A. M.; Castaneda-Miranda, E.; Castillo Gimenez, V.; Castro, N. F.; Cataldi, G.; Cataneo, F.; Catinaccio, A.; Catmore, J. R.; Cattai, A.; Cattani, G.; Caughron, S.; Cauz, D.; Cavalleri, P.; Cavalli, D.; Cavalli-Sforza, M.; Cavasinni, V.; Ceradini, F.; Cerqueira, A. S.; Cerri, A.; Cerrito, L.; Cerutti, F.; Cetin, S. A.; Cevenini, F.; Chafaq, A.; Chakraborty, D.; Chan, K.; Chapleau, B.; Chapman, J. D.; Chapman, J. W.; Chareyre, E.; Charlton, D. G.; Chavda, V.; Barajas, C. A. Chavez; Cheatham, S.; Chekanov, S.; Chekulaev, S. V.; Chelkov, G. A.; Chelstowska, M. A.; Chen, C.; Chen, H.; Chen, S.; Chen, T.; Chen, X.; Cheng, S.; Cheplakov, A.; Chepurnov, V. F.; Cherkaoui El Moursli, R.; Chernyatin, V.; Cheu, E.; Cheung, S. L.; Chevalier, L.; Chiefari, G.; Chikovani, L.; Childers, J. T.; Chilingarov, A.; Chiodini, G.; Chizhov, M. V.; Choudalakis, G.; Chouridou, S.; Christidi, I. A.; Christov, A.; Chromek-Burckhart, D.; Chu, M. L.; Chudoba, J.; Ciapetti, G.; Ciba, K.; Ciftci, A. K.; Ciftci, R.; Cinca, D.; Cindro, V.; Ciobotaru, M. D.; Ciocca, C.; Ciocio, A.; Cirilli, M.; Ciubancan, M.; Clark, A.; Clark, P. J.; Cleland, W.; Clemens, J. C.; Clement, B.; Clement, C.; Clifft, R. W.; Coadou, Y.; Cobal, M.; Coccaro, A.; Cochran, J.; Coe, P.; Cogan, J. G.; Coggeshall, J.; Cogneras, E.; Cojocaru, C. D.; Colas, J.; Colijn, A. P.; Collard, C.; Collins, N. J.; Collins-Tooth, C.; Collot, J.; Colon, G.; Conde Muiño, P.; Coniavitis, E.; Conidi, M. C.; Consonni, M.; Consorti, V.; Constantinescu, S.; Conta, C.; Conventi, F.; Cook, J.; Cooke, M.; Cooper, B. D.; Cooper-Sarkar, A. M.; Cooper-Smith, N. J.; Copic, K.; Cornelissen, T.; Corradi, M.; Corriveau, F.; Cortes-Gonzalez, A.; Cortiana, G.; Costa, G.; Costa, M. J.; Costanzo, D.; Costin, T.; Côté, D.; Coura Torres, R.; Courneyea, L.; Cowan, G.; Cowden, C.; Cox, B. E.; Cranmer, K.; Crescioli, F.; Cristinziani, M.; Crosetti, G.; Crupi, R.; Crépé-Renaudin, S.; Cuciuc, C.-M.; Almenar, C. Cuenca; Cuhadar Donszelmann, T.; Cuneo, S.; Curatolo, M.; Curtis, C. J.; Cwetanski, P.; Czirr, H.; Czyczula, Z.; D'Auria, S.; D'Onofrio, M.; D'Orazio, A.; Da Silva, P. V. M.; Da Via, C.; Dabrowski, W.; Dai, T.; Dallapiccola, C.; Dam, M.; Dameri, M.; Damiani, D. S.; Danielsson, H. O.; Dannheim, D.; Dao, V.; Darbo, G.; Darlea, G. L.; Daum, C.; Dauvergne, J. P.; Davey, W.; Davidek, T.; Davidson, N.; Davidson, R.; Davies, E.; Davies, M.; Davison, A. R.; Davygora, Y.; Dawe, E.; Dawson, I.; Dawson, J. W.; Daya, R. K.; De, K.; de Asmundis, R.; De Castro, S.; De Castro Faria Salgado, P. E.; De Cecco, S.; de Graat, J.; De Groot, N.; de Jong, P.; De La Taille, C.; De la Torre, H.; De Lotto, B.; De Mora, L.; De Nooij, L.; De Oliveira Branco, M.; De Pedis, D.; de Saintignon, P.; De Salvo, A.; De Sanctis, U.; De Santo, A.; De Vivie De Regie, J. B.; Dean, S.; Dedovich, D. V.; Degenhardt, J.; Dehchar, M.; Deile, M.; Del Papa, C.; Del Peso, J.; Del Prete, T.; Deliyergiyev, M.; Dell'Acqua, A.; Dell'Asta, L.; Della Pietra, M.; della Volpe, D.; Delmastro, M.; Delpierre, P.; Delruelle, N.; Delsart, P. A.; Deluca, C.; Demers, S.; Demichev, M.; Demirkoz, B.; Deng, J.; Denisov, S. P.; Derendarz, D.; Derkaoui, J. E.; Derue, F.; Dervan, P.; Desch, K.; Devetak, E.; Deviveiros, P. O.; Dewhurst, A.; DeWilde, B.; Dhaliwal, S.; Dhullipudi, R.; Di Ciaccio, A.; Di Ciaccio, L.; Di Girolamo, A.; Di Girolamo, B.; Di Luise, S.; Di Mattia, A.; Di Micco, B.; Di Nardo, R.; Di Simone, A.; Di Sipio, R.; Diaz, M. A.; Diblen, F.; Diehl, E. B.; Dietrich, J.; Dietzsch, T. A.; Diglio, S.; Dindar Yagci, K.; Dingfelder, J.; Dionisi, C.; Dita, P.; Dita, S.; Dittus, F.; Djama, F.; Djobava, T.; do Vale, M. A. B.; Do Valle Wemans, A.; Doan, T. K. O.; Dobbs, M.; Dobinson, R.; Dobos, D.; Dobson, E.; Dobson, M.; Dodd, J.; Doglioni, C.; Doherty, T.; Doi, Y.; Dolejsi, J.; Dolenc, I.; Dolezal, Z.; Dolgoshein, B. A.; Dohmae, T.; Donadelli, M.; Donega, M.; Donini, J.; Dopke, J.; Doria, A.; Dos Anjos, A.; Dosil, M.; Dotti, A.; Dova, M. T.; Dowell, J. D.; Doxiadis, A. D.; Doyle, A. T.; Drasal, Z.; Drees, J.; Dressnandt, N.; Drevermann, H.; Driouichi, C.; Dris, M.; Dubbert, J.; Dubbs, T.; Dube, S.; Duchovni, E.; Duckeck, G.; Dudarev, A.; Dudziak, F.; Dührssen, M.; Duerdoth, I. P.; Duflot, L.; Dufour, M.-A.; Dunford, M.; Yildiz, H. Duran; Duxfield, R.; Dwuznik, M.; Dydak, F.; Dzahini, D.; Düren, M.; Ebenstein, W. L.; Ebke, J.; Eckert, S.; Eckweiler, S.; Edmonds, K.; Edwards, C. A.; Edwards, N. C.; Ehrenfeld, W.; Ehrich, T.; Eifert, T.; Eigen, G.; Einsweiler, K.; Eisenhandler, E.; Ekelof, T.; El Kacimi, M.; Ellert, M.; Elles, S.; Ellinghaus, F.; Ellis, K.; Ellis, N.; Elmsheuser, J.; Elsing, M.; Ely, R.; Emeliyanov, D.; Engelmann, R.; Engl, A.; Epp, B.; Eppig, A.; Erdmann, J.; Ereditato, A.; Eriksson, D.; Ernst, J.; Ernst, M.; Ernwein, J.; Errede, D.; Errede, S.; Ertel, E.; Escalier, M.; Escobar, C.; Espinal Curull, X.; Esposito, B.; Etienne, F.; Etienvre, A. I.; Etzion, E.; Evangelakou, D.; Evans, H.; Fabbri, L.; Fabre, C.; Fakhrutdinov, R. M.; Falciano, S.; Fang, Y.; Fanti, M.; Farbin, A.; Farilla, A.; Farley, J.; Farooque, T.; Farrington, S. M.; Farthouat, P.; Fassnacht, P.; Fassouliotis, D.; Fatholahzadeh, B.; Favareto, A.; Fayard, L.; Fazio, S.; Febbraro, R.; Federic, P.; Fedin, O. L.; Fedorko, W.; Fehling-Kaschek, M.; Feligioni, L.; Fellmann, D.; Felzmann, C. U.; Feng, C.; Feng, E. J.; Fenyuk, A. B.; Ferencei, J.; Ferland, J.; Fernando, W.; Ferrag, S.; Ferrando, J.; Ferrara, V.; Ferrari, A.; Ferrari, P.; Ferrari, R.; Ferrer, A.; Ferrer, M. L.; Ferrere, D.; Ferretti, C.; Ferretto Parodi, A.; Fiascaris, M.; Fiedler, F.; Filipčič, A.; Filippas, A.; Filthaut, F.; Fincke-Keeler, M.; Fiolhais, M. C. N.; Fiorini, L.; Firan, A.; Fischer, G.; Fischer, P.; Fisher, M. J.; Fisher, S. M.; Flechl, M.; Fleck, I.; Fleckner, J.; Fleischmann, P.; Fleischmann, S.; Flick, T.; Flores Castillo, L. R.; Flowerdew, M. J.; Föhlisch, F.; Fokitis, M.; Martin, T. Fonseca; Forbush, D. A.; Formica, A.; Forti, A.; Fortin, D.; Foster, J. M.; Fournier, D.; Foussat, A.; Fowler, A. J.; Fowler, K.; Fox, H.; Francavilla, P.; Franchino, S.; Francis, D.; Frank, T.; Franklin, M.; Franz, S.; Fraternali, M.; Fratina, S.; French, S. T.; Froeschl, R.; Froidevaux, D.; Frost, J. A.; Fukunaga, C.; Fullana Torregrosa, E.; Fuster, J.; Gabaldon, C.; Gabizon, O.; Gadfort, T.; Gadomski, S.; Gagliardi, G.; Gagnon, P.; Galea, C.; Gallas, E. J.; Gallas, M. V.; Gallo, V.; Gallop, B. J.; Gallus, P.; Galyaev, E.; Gan, K. K.; Gao, Y. S.; Gapienko, V. A.; Gaponenko, A.; Garberson, F.; Garcia-Sciveres, M.; García, C.; Navarro, J. E. García; Gardner, R. W.; Garelli, N.; Garitaonandia, H.; Garonne, V.; Garvey, J.; Gatti, C.; Gaudio, G.; Gaumer, O.; Gaur, B.; Gauthier, L.; Gavrilenko, I. L.; Gay, C.; Gaycken, G.; Gayde, J.-C.; Gazis, E. N.; Ge, P.; Gee, C. N. P.; Geerts, D. A. A.; Geich-Gimbel, Ch.; Gellerstedt, K.; Gemme, C.; Gemmell, A.; Genest, M. H.; Gentile, S.; George, M.; George, S.; Gerlach, P.; Gershon, A.; Geweniger, C.; Ghazlane, H.; Ghez, P.; Ghodbane, N.; Giacobbe, B.; Giagu, S.; Giakoumopoulou, V.; Giangiobbe, V.; Gianotti, F.; Gibbard, B.; Gibson, A.; Gibson, S. M.; Gilbert, L. M.; Gilchriese, M.; Gilewsky, V.; Gillberg, D.; Gillman, A. R.; Gingrich, D. M.; Ginzburg, J.; Giokaris, N.; Giordano, R.; Giorgi, F. M.; Giovannini, P.; Giraud, P. F.; Giugni, D.; Giunta, M.; Giusti, P.; Gjelsten, B. K.; Gladilin, L. K.; Glasman, C.; Glatzer, J.; Glazov, A.; Glitza, K. W.; Glonti, G. L.; Godfrey, J.; Godlewski, J.; Goebel, M.; Göpfert, T.; Goeringer, C.; Gössling, C.; Göttfert, T.; Goldfarb, S.; Goldin, D.; Golling, T.; Golovnia, S. N.; Gomes, A.; Fajardo, L. S. Gomez; Gonçalo, R.; Goncalves Pinto Firmino Da Costa, J.; Gonella, L.; Gonidec, A.; Gonzalez, S.; González de la Hoz, S.; Silva, M. L. Gonzalez; Gonzalez-Sevilla, S.; Goodson, J. J.; Goossens, L.; Gorbounov, P. A.; Gordon, H. A.; Gorelov, I.; Gorfine, G.; Gorini, B.; Gorini, E.; Gorišek, A.; Gornicki, E.; Gorokhov, S. A.; Goryachev, V. N.; Gosdzik, B.; Gosselink, M.; Gostkin, M. I.; Gouanère, M.; Eschrich, I. Gough; Gouighri, M.; Goujdami, D.; Goulette, M. P.; Goussiou, A. G.; Goy, C.; Grabowska-Bold, I.; Grabski, V.; Grafström, P.; Grah, C.; Grahn, K.-J.; Grancagnolo, F.; Grancagnolo, S.; Grassi, V.; Gratchev, V.; Grau, N.; Gray, H. M.; Gray, J. A.; Graziani, E.; Grebenyuk, O. G.; Greenfield, D.; Greenshaw, T.; Greenwood, Z. D.; Gregor, I. M.; Grenier, P.; Griffiths, J.; Grigalashvili, N.; Grillo, A. A.; Grinstein, S.; Grishkevich, Y. V.; Grivaz, J.-F.; Grognuz, J.; Groh, M.; Gross, E.; Grosse-Knetter, J.; Groth-Jensen, J.; Grybel, K.; Guarino, V. J.; Guest, D.; Guicheney, C.; Guida, A.; Guillemin, T.; Guindon, S.; Guler, H.; Gunther, J.; Guo, B.; Guo, J.; Gupta, A.; Gusakov, Y.; Gushchin, V. N.; Gutierrez, A.; Gutierrez, P.; Guttman, N.; Gutzwiller, O.; Guyot, C.; Gwenlan, C.; Gwilliam, C. B.; Haas, A.; Haas, S.; Haber, C.; Hackenburg, R.; Hadavand, H. K.; Hadley, D. R.; Haefner, P.; Hahn, F.; Haider, S.; Hajduk, Z.; Hakobyan, H.; Haller, J.; Hamacher, K.; Hamal, P.; Hamilton, A.; Hamilton, S.; Han, H.; Han, L.; Hanagaki, K.; Hance, M.; Handel, C.; Hanke, P.; Hansen, J. R.; Hansen, J. B.; Hansen, J. D.; Hansen, P. H.; Hansson, P.; Hara, K.; Hare, G. A.; Harenberg, T.; Harkusha, S.; Harper, D.; Harrington, R. D.; Harris, O. M.; Harrison, K.; Hartert, J.; Hartjes, F.; Haruyama, T.; Harvey, A.; Hasegawa, S.; Hasegawa, Y.; Hassani, S.; Hatch, M.; Hauff, D.; Haug, S.; Hauschild, M.; Hauser, R.; Havranek, M.; Hawes, B. M.; Hawkes, C. M.; Hawkings, R. J.; Hawkins, D.; Hayakawa, T.; Hayden, D.; Hayward, H. S.; Haywood, S. J.; Hazen, E.; He, M.; Head, S. J.; Hedberg, V.; Heelan, L.; Heim, S.; Heinemann, B.; Heisterkamp, S.; Helary, L.; Heller, M.; Hellman, S.; Hellmich, D.; Helsens, C.; Henderson, R. C. W.; Henke, M.; Henrichs, A.; Correia, A. M. Henriques; Henrot-Versille, S.; Henry-Couannier, F.; Hensel, C.; Henß, T.; Hernandez, C. M.; Hernández Jiménez, Y.; Herrberg, R.; Hershenhorn, A. D.; Herten, G.; Hertenberger, R.; Hervas, L.; Hessey, N. P.; Hidvegi, A.; Higón-Rodriguez, E.; Hill, D.; Hill, J. C.; Hill, N.; Hiller, K. H.; Hillert, S.; Hillier, S. J.; Hinchliffe, I.; Hines, E.; Hirose, M.; Hirsch, F.; Hirschbuehl, D.; Hobbs, J.; Hod, N.; Hodgkinson, M. C.; Hodgson, P.; Hoecker, A.; Hoeferkamp, M. R.; Hoffman, J.; Hoffmann, D.; Hohlfeld, M.; Holder, M.; Holmes, A.; Holmgren, S. O.; Holy, T.; Holzbauer, J. L.; Homma, Y.; Hong, T. M.; Hooft van Huysduynen, L.; Horazdovsky, T.; Horn, C.; Horner, S.; Horton, K.; Hostachy, J.-Y.; Hou, S.; Houlden, M. A.; Hoummada, A.; Howarth, J.; Howell, D. F.; Hristova, I.; Hrivnac, J.; Hruska, I.; Hryn'ova, T.; Hsu, P. J.; Hsu, S.-C.; Huang, G. S.; Hubacek, Z.; Hubaut, F.; Huegging, F.; Huffman, T. B.; Hughes, E. W.; Hughes, G.; Hughes-Jones, R. E.; Huhtinen, M.; Hurst, P.; Hurwitz, M.; Husemann, U.; Huseynov, N.; Huston, J.; Huth, J.; Iacobucci, G.; Iakovidis, G.; Ibbotson, M.; Ibragimov, I.; Ichimiya, R.; Iconomidou-Fayard, L.; Idarraga, J.; Idzik, M.; Iengo, P.; Igonkina, O.; Ikegami, Y.; Ikeno, M.; Ilchenko, Y.; Iliadis, D.; Imbault, D.; Imhaeuser, M.; Imori, M.; Ince, T.; Inigo-Golfin, J.; Ioannou, P.; Iodice, M.; Ionescu, G.; Irles Quiles, A.; Ishii, K.; Ishikawa, A.; Ishino, M.; Ishmukhametov, R.; Issever, C.; Istin, S.; Itoh, Y.; Ivashin, A. V.; Iwanski, W.; Iwasaki, H.; Izen, J. M.; Izzo, V.; Jackson, B.; Jackson, J. N.; Jackson, P.; Jaekel, M. R.; Jain, V.; Jakobs, K.; Jakobsen, S.; Jakubek, J.; Jana, D. K.; Jankowski, E.; Jansen, E.; Jantsch, A.; Janus, M.; Jarlskog, G.; Jeanty, L.; Jelen, K.; Jen-La Plante, I.; Jenni, P.; Jeremie, A.; Jež, P.; Jézéquel, S.; Jha, M. K.; Ji, H.; Ji, W.; Jia, J.; Jiang, Y.; Belenguer, M. Jimenez; Jin, G.; Jin, S.; Jinnouchi, O.; Joergensen, M. D.; Joffe, D.; Johansen, L. G.; Johansen, M.; Johansson, K. E.; Johansson, P.; Johnert, S.; Johns, K. A.; Jon-And, K.; Jones, G.; Jones, R. W. L.; Jones, T. W.; Jones, T. J.; Jonsson, O.; Joram, C.; Jorge, P. M.; Joseph, J.; Jovin, T.; Ju, X.; Juranek, V.; Jussel, P.; Kabachenko, V. V.; Kabana, S.; Kaci, M.; Kaczmarska, A.; Kadlecik, P.; Kado, M.; Kagan, H.; Kagan, M.; Kaiser, S.; Kajomovitz, E.; Kalinin, S.; Kalinovskaya, L. V.; Kama, S.; Kanaya, N.; Kaneda, M.; Kanno, T.; Kantserov, V. A.; Kanzaki, J.; Kaplan, B.; Kapliy, A.; Kaplon, J.; Kar, D.; Karagoz, M.; Karnevskiy, M.; Karr, K.; Kartvelishvili, V.; Karyukhin, A. N.; Kashif, L.; Kasmi, A.; Kass, R. D.; Kastanas, A.; Kataoka, M.; Kataoka, Y.; Katsoufis, E.; Katzy, J.; Kaushik, V.; Kawagoe, K.; Kawamoto, T.; Kawamura, G.; Kayl, M. S.; Kazanin, V. A.; Kazarinov, M. Y.; Keates, J. R.; Keeler, R.; Kehoe, R.; Keil, M.; Kekelidze, G. D.; Kelly, M.; Kennedy, J.; Kenney, C. J.; Kenyon, M.; Kepka, O.; Kerschen, N.; Kerševan, B. P.; Kersten, S.; Kessoku, K.; Ketterer, C.; Keung, J.; Khakzad, M.; Khalil-zada, F.; Khandanyan, H.; Khanov, A.; Kharchenko, D.; Khodinov, A.; Kholodenko, A. G.; Khomich, A.; Khoo, T. J.; Khoriauli, G.; Khoroshilov, A.; Khovanskiy, N.; Khovanskiy, V.; Khramov, E.; Khubua, J.; Kim, H.; Kim, M. S.; Kim, P. C.; Kim, S. H.; Kimura, N.; Kind, O.; King, B. T.; King, M.; King, R. S. B.; Kirk, J.; Kirsch, G. P.; Kirsch, L. E.; Kiryunin, A. E.; Kisielewska, D.; Kittelmann, T.; Kiver, A. M.; Kiyamura, H.; Kladiva, E.; Klaiber-Lodewigs, J.; Klein, M.; Klein, U.; Kleinknecht, K.; Klemetti, M.; Klier, A.; Klimentov, A.; Klingenberg, R.; Klinkby, E. B.; Klioutchnikova, T.; Klok, P. F.; Klous, S.; Kluge, E.-E.; Kluge, T.; Kluit, P.; Kluth, S.; Kneringer, E.; Knobloch, J.; Knoops, E. B. F. G.; Knue, A.; Ko, B. R.; Kobayashi, T.; Kobel, M.; Kocian, M.; Kocnar, A.; Kodys, P.; Köneke, K.; König, A. C.; Koenig, S.; Köpke, L.; Koetsveld, F.; Koevesarki, P.; Koffas, T.; Koffeman, E.; Kohn, F.; Kohout, Z.; Kohriki, T.; Koi, T.; Kokott, T.; Kolachev, G. M.; Kolanoski, H.; Kolesnikov, V.; Koletsou, I.; Koll, J.; Kollar, D.; Kollefrath, M.; Kolya, S. D.; Komar, A. A.; Komaragiri, J. R.; Komori, Y.; Kondo, T.; Kono, T.; Kononov, A. I.; Konoplich, R.; Konstantinidis, N.; Kootz, A.; Koperny, S.; Kopikov, S. V.; Korcyl, K.; Kordas, K.; Koreshev, V.; Korn, A.; Korol, A.; Korolkov, I.; Korolkova, E. V.; Korotkov, V. A.; Kortner, O.; Kortner, S.; Kostyukhin, V. V.; Kotamäki, M. J.; Kotov, S.; Kotov, V. M.; Kotwal, A.; Kourkoumelis, C.; Kouskoura, V.; Koutsman, A.; Kowalewski, R.; Kowalski, T. Z.; Kozanecki, W.; Kozhin, A. S.; Kral, V.; Kramarenko, V. A.; Kramberger, G.; Krasel, O.; Krasny, M. W.; Krasznahorkay, A.; Kraus, J.; Kreisel, A.; Krejci, F.; Kretzschmar, J.; Krieger, N.; Krieger, P.; Kroeninger, K.; Kroha, H.; Kroll, J.; Kroseberg, J.; Krstic, J.; Kruchonak, U.; Krüger, H.; Kruker, T.; Krumshteyn, Z. V.; Kruth, A.; Kubota, T.; Kuehn, S.; Kugel, A.; Kuhl, T.; Kuhn, D.; Kukhtin, V.; Kulchitsky, Y.; Kuleshov, S.; Kummer, C.; Kuna, M.; Kundu, N.; Kunkle, J.; Kupco, A.; Kurashige, H.; Kurata, M.; Kurochkin, Y. A.; Kus, V.; Kuykendall, W.; Kuze, M.; Kuzhir, P.; Kvasnicka, O.; Kvita, J.; Kwee, R.; La Rosa, A.; La Rotonda, L.; Labarga, L.; Labbe, J.; Lablak, S.; Lacasta, C.; Lacava, F.; Lacker, H.; Lacour, D.; Lacuesta, V. R.; Ladygin, E.; Lafaye, R.; Laforge, B.; Lagouri, T.; Lai, S.; Laisne, E.; Lamanna, M.; Lampen, C. L.; Lampl, W.; Lancon, E.; Landgraf, U.; Landon, M. P. J.; Landsman, H.; Lane, J. L.; Lange, C.; Lankford, A. J.; Lanni, F.; Lantzsch, K.; Laplace, S.; Lapoire, C.; Laporte, J. F.; Lari, T.; Larionov, A. V.; Larner, A.; Lasseur, C.; Lassnig, M.; Lau, W.; Laurelli, P.; Lavorato, A.; Lavrijsen, W.; Laycock, P.; Lazarev, A. B.; Lazzaro, A.; Le Dortz, O.; Le Guirriec, E.; Le Maner, C.; Le Menedeu, E.; Lebel, C.; LeCompte, T.; Ledroit-Guillon, F.; Lee, H.; Lee, J. S. H.; Lee, S. C.; Lee, L.; Lefebvre, M.; Legendre, M.; Leger, A.; LeGeyt, B. C.; Legger, F.; Leggett, C.; Lehmacher, M.; Lehmann Miotto, G.; Lei, X.; Leite, M. A. L.; Leitner, R.; Lellouch, D.; Lellouch, J.; Leltchouk, M.; Lendermann, V.; Leney, K. J. C.; Lenz, T.; Lenzen, G.; Lenzi, B.; Leonhardt, K.; Leontsinis, S.; Leroy, C.; Lessard, J.-R.; Lesser, J.; Lester, C. G.; Leung Fook Cheong, A.; Levêque, J.; Levin, D.; Levinson, L. J.; Levitski, M. S.; Lewandowska, M.; Lewis, A.; Lewis, G. H.; Leyko, A. M.; Leyton, M.; Li, B.; Li, H.; Li, S.; Li, X.; Liang, Z.; Liang, Z.; Liberti, B.; Lichard, P.; Lichtnecker, M.; Lie, K.; Liebig, W.; Lifshitz, R.; Lilley, J. N.; Limbach, C.; Limosani, A.; Limper, M.; Lin, S. C.; Linde, F.; Linnemann, J. T.; Lipeles, E.; Lipinsky, L.; Lipniacka, A.; Liss, T. M.; Lissauer, D.; Lister, A.; Litke, A. M.; Liu, C.; Liu, D.; Liu, H.; Liu, J. B.; Liu, M.; Liu, S.; Liu, Y.; Livan, M.; Livermore, S. S. A.; Lleres, A.; Llorente Merino, J.; Lloyd, S. L.; Lobodzinska, E.; Loch, P.; Lockman, W. S.; Lockwitz, S.; Loddenkoetter, T.; Loebinger, F. K.; Loginov, A.; Loh, C. W.; Lohse, T.; Lohwasser, K.; Lokajicek, M.; Loken, J.; Lombardo, V. P.; Long, R. E.; Lopes, L.; Lopez Mateos, D.; Losada, M.; Loscutoff, P.; Sterzo, F. Lo; Losty, M. J.; Lou, X.; Lounis, A.; Loureiro, K. F.; Love, J.; Love, P. A.; Lowe, A. J.; Lu, F.; Lubatti, H. J.; Luci, C.; Lucotte, A.; Ludwig, A.; Ludwig, D.; Ludwig, I.; Ludwig, J.; Luehring, F.; Luijckx, G.; Lumb, D.; Luminari, L.; Lund, E.; Lund-Jensen, B.; Lundberg, B.; Lundberg, J.; Lundquist, J.; Lungwitz, M.; Lupi, A.; Lutz, G.; Lynn, D.; Lys, J.; Lytken, E.; Ma, H.; Ma, L. L.; Macana Goia, J. A.; Maccarrone, G.; Macchiolo, A.; Maček, B.; Miguens, J. Machado; Macina, D.; Mackeprang, R.; Madaras, R. J.; Mader, W. F.; Maenner, R.; Maeno, T.; Mättig, P.; Mättig, S.; Magalhaes Martins, P. J.; Magnoni, L.; Magradze, E.; Mahalalel, Y.; Mahboubi, K.; Mahout, G.; Maiani, C.; Maidantchik, C.; Maio, A.; Majewski, S.; Makida, Y.; Makovec, N.; Mal, P.; Malecki, Pa.; Malecki, P.; Maleev, V. P.; Malek, F.; Mallik, U.; Malon, D.; Maltezos, S.; Malyshev, V.; Malyukov, S.; Mameghani, R.; Mamuzic, J.; Manabe, A.; Mandelli, L.; Mandić, I.; Mandrysch, R.; Maneira, J.; Mangeard, P. S.; Manjavidze, I. D.; Mann, A.; Manning, P. M.; Manousakis-Katsikakis, A.; Mansoulie, B.; Manz, A.; Mapelli, A.; Mapelli, L.; March, L.; Marchand, J. F.; Marchese, F.; Marchiori, G.; Marcisovsky, M.; Marin, A.; Marino, C. P.; Marroquim, F.; Marshall, R.; Marshall, Z.; Martens, F. K.; Marti-Garcia, S.; Martin, A. J.; Martin, B.; Martin, B.; Martin, F. F.; Martin, J. P.; Martin, Ph.; Martin, T. A.; dit Latour, B. Martin; Martinez, M.; Outschoorn, V. Martinez; Martyniuk, A. C.; Marx, M.; Marzano, F.; Marzin, A.; Masetti, L.; Mashimo, T.; Mashinistov, R.; Masik, J.; Maslennikov, A. L.; Maß, M.; Massa, I.; Massaro, G.; Massol, N.; Mastrandrea, P.; Mastroberardino, A.; Masubuchi, T.; Mathes, M.; Matricon, P.; Matsumoto, H.; Matsunaga, H.; Matsushita, T.; Mattravers, C.; Maugain, J. M.; Maxfield, S. J.; Maximov, D. A.; May, E. N.; Mayne, A.; Mazini, R.; Mazur, M.; Mazzanti, M.; Mazzoni, E.; Mc Kee, S. P.; McCarn, A.; McCarthy, R. L.; McCarthy, T. G.; McCubbin, N. A.; McFarlane, K. W.; Mcfayden, J. A.; McGlone, H.; Mchedlidze, G.; McLaren, R. A.; Mclaughlan, T.; McMahon, S. J.; McPherson, R. A.; Meade, A.; Mechnich, J.; Mechtel, M.; Medinnis, M.; Meera-Lebbai, R.; Meguro, T.; Mehdiyev, R.; Mehlhase, S.; Mehta, A.; Meier, K.; Meinhardt, J.; Meirose, B.; Melachrinos, C.; Mellado Garcia, B. R.; Mendoza Navas, L.; Meng, Z.; Mengarelli, A.; Menke, S.; Menot, C.; Meoni, E.; Mercurio, K. M.; Mermod, P.; Merola, L.; Meroni, C.; Merritt, F. S.; Messina, A.; Metcalfe, J.; Mete, A. S.; Meuser, S.; Meyer, C.; Meyer, J.-P.; Meyer, J.; Meyer, J.; Meyer, T. C.; Meyer, W. T.; Miao, J.; Michal, S.; Micu, L.; Middleton, R. P.; Miele, P.; Migas, S.; Mijović, L.; Mikenberg, G.; Mikestikova, M.; Mikuž, M.; Miller, D. W.; Miller, R. J.; Mills, W. J.; Mills, C.; Milov, A.; Milstead, D. A.; Milstein, D.; Minaenko, A. A.; Miñano, M.; Minashvili, I. A.; Mincer, A. I.; Mindur, B.; Mineev, M.; Ming, Y.; Mir, L. M.; Mirabelli, G.; Miralles Verge, L.; Misiejuk, A.; Mitrevski, J.; Mitrofanov, G. Y.; Mitsou, V. A.; Mitsui, S.; Miyagawa, P. S.; Miyazaki, K.; Mjörnmark, J. U.; Moa, T.; Mockett, P.; Moed, S.; Moeller, V.; Mönig, K.; Möser, N.; Mohapatra, S.; Mohn, B.; Mohr, W.; Mohrdieck-Möck, S.; Moisseev, A. M.; Moles-Valls, R.; Molina-Perez, J.; Monk, J.; Monnier, E.; Montesano, S.; Monticelli, F.; Monzani, S.; Moore, R. W.; Moorhead, G. F.; Herrera, C. Mora; Moraes, A.; Morais, A.; Morange, N.; Morel, J.; Morello, G.; Moreno, D.; Llácer, M. Moreno; Morettini, P.; Morii, M.; Morin, J.; Morita, Y.; Morley, A. K.; Mornacchi, G.; Morone, M.-C.; Morozov, S. V.; Morris, J. D.; Morvaj, L.; Moser, H. G.; Mosidze, M.; Moss, J.; Mount, R.; Mountricha, E.; Mouraviev, S. V.; Moyse, E. J. W.; Mudrinic, M.; Mueller, F.; Mueller, J.; Mueller, K.; Müller, T. A.; Muenstermann, D.; Muijs, A.; Muir, A.; Munwes, Y.; Murakami, K.; Murray, W. J.; Mussche, I.; Musto, E.; Myagkov, A. G.; Myska, M.; Nadal, J.; Nagai, K.; Nagano, K.; Nagasaka, Y.; Nairz, A. M.; Nakahama, Y.; Nakamura, K.; Nakano, I.; Nanava, G.; Napier, A.; Nash, M.; Nation, N. R.; Nattermann, T.; Naumann, T.; Navarro, G.; Neal, H. A.; Nebot, E.; Nechaeva, P. Yu.; Negri, A.; Negri, G.; Nektarijevic, S.; Nelson, A.; Nelson, S.; Nelson, T. K.; Nemecek, S.; Nemethy, P.; Nepomuceno, A. A.; Nessi, M.; Nesterov, S. Y.; Neubauer, M. S.; Neusiedl, A.; Neves, R. M.; Nevski, P.; Newman, P. R.; Nickerson, R. B.; Nicolaidou, R.; Nicolas, L.; Nicquevert, B.; Niedercorn, F.; Nielsen, J.; Niinikoski, T.; Nikiforov, A.; Nikolaenko, V.; Nikolaev, K.; Nikolic-Audit, I.; Nikolics, K.; Nikolopoulos, K.; Nilsen, H.; Nilsson, P.; Ninomiya, Y.; Nisati, A.; Nishiyama, T.; Nisius, R.; Nodulman, L.; Nomachi, M.; Nomidis, I.; Nordberg, M.; Nordkvist, B.; Norton, P. R.; Novakova, J.; Nozaki, M.; Nožička, M.; Nozka, L.; Nugent, I. M.; Nuncio-Quiroz, A.-E.; Nunes Hanninger, G.; Nunnemann, T.; Nurse, E.; Nyman, T.; O'Brien, B. J.; O'Neale, S. W.; O'Neil, D. C.; O'Shea, V.; Oakham, F. G.; Oberlack, H.; Ocariz, J.; Ochi, A.; Oda, S.; Odaka, S.; Odier, J.; Ogren, H.; Oh, A.; Oh, S. H.; Ohm, C. C.; Ohshima, T.; Ohshita, H.; Ohska, T. K.; Ohsugi, T.; Okada, S.; Okawa, H.; Okumura, Y.; Okuyama, T.; Olcese, M.; Olchevski, A. G.; Oliveira, M.; Damazio, D. Oliveira; Garcia, E. Oliver; Olivito, D.; Olszewski, A.; Olszowska, J.; Omachi, C.; Onofre, A.; Onyisi, P. U. E.; Oram, C. J.; Oreglia, M. J.; Oren, Y.; Orestano, D.; Orlov, I.; Oropeza Barrera, C.; Orr, R. S.; Osculati, B.; Ospanov, R.; Osuna, C.; y Garzon, G. Otero; Ottersbach, J. P.; Ouchrif, M.; Ould-Saada, F.; Ouraou, A.; Ouyang, Q.; Owen, M.; Owen, S.; Øye, O. K.; Ozcan, V. E.; Ozturk, N.; Pacheco Pages, A.; Padilla Aranda, C.; Griso, S. Pagan; Paganis, E.; Paige, F.; Pajchel, K.; Palestini, S.; Pallin, D.; Palma, A.; Palmer, J. D.; Pan, Y. B.; Panagiotopoulou, E.; Panes, B.; Panikashvili, N.; Panitkin, S.; Pantea, D.; Panuskova, M.; Paolone, V.; Papadelis, A.; Papadopoulou, Th. D.; Paramonov, A.; Park, W.; Parker, M. A.; Parodi, F.; Parsons, J. A.; Parzefall, U.; Pasqualucci, E.; Passeri, A.; Pastore, F.; Pastore, Fr.; Pásztor, G.; Pataraia, S.; Patel, N.; Pater, J. R.; Patricelli, S.; Pauly, T.; Pecsy, M.; Pedraza Morales, M. I.; Peleganchuk, S. V.; Peng, H.; Pengo, R.; Penson, A.; Penwell, J.; Perantoni, M.; Perez, K.; Cavalcanti, T. Perez; Codina, E. Perez; Pérez García-Estañ, M. T.; Reale, V. Perez; Perini, L.; Pernegger, H.; Perrino, R.; Perrodo, P.; Persembe, S.; Peshekhonov, V. D.; Peters, O.; Petersen, B. A.; Petersen, J.; Petersen, T. C.; Petit, E.; Petridis, A.; Petridou, C.; Petrolo, E.; Petrucci, F.; Petschull, D.; Petteni, M.; Pezoa, R.; Phan, A.; Phillips, A. W.; Phillips, P. W.; Piacquadio, G.; Piccaro, E.; Piccinini, M.; Pickford, A.; Piec, S. M.; Piegaia, R.; Pilcher, J. E.; Pilkington, A. D.; Pina, J.; Pinamonti, M.; Pinder, A.; Pinfold, J. L.; Ping, J.; Pinto, B.; Pirotte, O.; Pizio, C.; Placakyte, R.; Plamondon, M.; Plano, W. G.; Pleier, M.-A.; Pleskach, A. V.; Poblaguev, A.; Poddar, S.; Podlyski, F.; Poggioli, L.; Poghosyan, T.; Pohl, M.; Polci, F.; Polesello, G.; Policicchio, A.; Polini, A.; Poll, J.; Polychronakos, V.; Pomarede, D. M.; Pomeroy, D.; Pommès, K.; Pontecorvo, L.; Pope, B. G.; Popeneciu, G. A.; Popovic, D. S.; Poppleton, A.; Portell Bueso, X.; Porter, R.; Posch, C.; Pospelov, G. E.; Pospisil, S.; Potrap, I. N.; Potter, C. J.; Potter, C. T.; Poulard, G.; Poveda, J.; Prabhu, R.; Pralavorio, P.; Prasad, S.; Pravahan, R.; Prell, S.; Pretzl, K.; Pribyl, L.; Price, D.; Price, L. E.; Price, M. J.; Prichard, P. M.; Prieur, D.; Primavera, M.; Prokofiev, K.; Prokoshin, F.; Protopopescu, S.; Proudfoot, J.; Prudent, X.; Przysiezniak, H.; Psoroulas, S.; Ptacek, E.; Purdham, J.; Purohit, M.; Puzo, P.; Pylypchenko, Y.; Qian, J.; Qian, Z.; Qin, Z.; Quadt, A.; Quarrie, D. R.; Quayle, W. B.; Quinonez, F.; Raas, M.; Radescu, V.; Radics, B.; Rador, T.; Ragusa, F.; Rahal, G.; Rahimi, A. M.; Rahm, D.; Rajagopalan, S.; Rammensee, M.; Rammes, M.; Ramstedt, M.; Randrianarivony, K.; Ratoff, P. N.; Rauscher, F.; Rauter, E.; Raymond, M.; Read, A. L.; Rebuzzi, D. M.; Redelbach, A.; Redlinger, G.; Reece, R.; Reeves, K.; Reichold, A.; Reinherz-Aronis, E.; Reinsch, A.; Reisinger, I.; Reljic, D.; Rembser, C.; Ren, Z. L.; Renaud, A.; Renkel, P.; Rescigno, M.; Resconi, S.; Resende, B.; Reznicek, P.; Rezvani, R.; Richards, A.; Richter, R.; Richter-Was, E.; Ridel, M.; Rieke, S.; Rijpstra, M.; Rijssenbeek, M.; Rimoldi, A.; Rinaldi, L.; Rios, R. R.; Riu, I.; Rivoltella, G.; Rizatdinova, F.; Rizvi, E.; Robertson, S. H.; Robichaud-Veronneau, A.; Robinson, D.; Robinson, J. E. M.; Robinson, M.; Robson, A.; Rocha de Lima, J. G.; Roda, C.; Dos Santos, D. Roda; Rodier, S.; Rodriguez, D.; Garcia, Y. Rodriguez; Roe, A.; Roe, S.; Røhne, O.; Rojo, V.; Rolli, S.; Romaniouk, A.; Romanov, V. M.; Romeo, G.; Romero Maltrana, D.; Roos, L.; Ros, E.; Rosati, S.; Rosbach, K.; Rose, M.; Rosenbaum, G. A.; Rosenberg, E. I.; Rosendahl, P. L.; Rosselet, L.; Rossetti, V.; Rossi, E.; Rossi, L. P.; Rossi, L.; Rotaru, M.; Roth, I.; Rothberg, J.; Rousseau, D.; Royon, C. R.; Rozanov, A.; Rozen, Y.; Ruan, X.; Rubinskiy, I.; Ruckert, B.; Ruckstuhl, N.; Rud, V. I.; Rudolph, C.; Rudolph, G.; Rühr, F.; Ruggieri, F.; Ruiz-Martinez, A.; Rulikowska-Zarebska, E.; Rumiantsev, V.; Rumyantsev, L.; Runge, K.; Runolfsson, O.; Rurikova, Z.; Rusakovich, N. A.; Rust, D. R.; Rutherfoord, J. P.; Ruwiedel, C.; Ruzicka, P.; Ryabov, Y. F.; Ryadovikov, V.; Ryan, P.; Rybar, M.; Rybkin, G.; Ryder, N. C.; Rzaeva, S.; Saavedra, A. F.; Sadeh, I.; Sadrozinski, H. F.-W.; Sadykov, R.; Safai Tehrani, F.; Sakamoto, H.; Salamanna, G.; Salamon, A.; Saleem, M.; Salihagic, D.; Salnikov, A.; Salt, J.; Salvachua Ferrando, B. M.; Salvatore, D.; Salvatore, F.; Salvucci, A.; Salzburger, A.; Sampsonidis, D.; Samset, B. H.; Sanchez, A.; Sandaker, H.; Sander, H. G.; Sanders, M. P.; Sandhoff, M.; Sandoval, T.; Sandstroem, R.; Sandvoss, S.; Sankey, D. P. C.; Sansoni, A.; Santamarina Rios, C.; Santoni, C.; Santonico, R.; Santos, H.; Saraiva, J. G.; Sarangi, T.; Sarkisyan-Grinbaum, E.; Sarri, F.; Sartisohn, G.; Sasaki, O.; Sasaki, T.; Sasao, N.; Satsounkevitch, I.; Sauvage, G.; Sauvan, E.; Sauvan, J. B.; Savard, P.; Savinov, V.; Savu, D. O.; Savva, P.; Sawyer, L.; Saxon, D. H.; Says, L. P.; Sbarra, C.; Sbrizzi, A.; Scallon, O.; Scannicchio, D. A.; Schaarschmidt, J.; Schacht, P.; Schäfer, U.; Schaepe, S.; Schaetzel, S.; Schaffer, A. C.; Schaile, D.; Schamberger, R. D.; Schamov, A. G.; Scharf, V.; Schegelsky, V. A.; Scheirich, D.; Scherzer, M. I.; Schiavi, C.; Schieck, J.; Schioppa, M.; Schlenker, S.; Schlereth, J. L.; Schmidt, E.; Schmieden, K.; Schmitt, C.; Schmitt, S.; Schmitz, M.; Schöning, A.; Schott, M.; Schouten, D.; Schovancova, J.; Schram, M.; Schroeder, C.; Schroer, N.; Schuh, S.; Schuler, G.; Schultes, J.; Schultz-Coulon, H.-C.; Schulz, H.; Schumacher, J. W.; Schumacher, M.; Schumm, B. A.; Schune, Ph.; Schwanenberger, C.; Schwartzman, A.; Schwemling, Ph.; Schwienhorst, R.; Schwierz, R.; Schwindling, J.; Schwindt, T.; Scott, W. G.; Searcy, J.; Sedykh, E.; Segura, E.; Seidel, S. C.; Seiden, A.; Seifert, F.; Seixas, J. M.; Sekhniaidze, G.; Seliverstov, D. M.; Sellden, B.; Sellers, G.; Seman, M.; Semprini-Cesari, N.; Serfon, C.; Serin, L.; Seuster, R.; Severini, H.; Sevior, M. E.; Sfyrla, A.; Shabalina, E.; Shamim, M.; Shan, L. Y.; Shank, J. T.; Shao, Q. T.; Shapiro, M.; Shatalov, P. B.; Shaver, L.; Shaw, C.; Shaw, K.; Sherman, D.; Sherwood, P.; Shibata, A.; Shichi, H.; Shimizu, S.; Shimojima, M.; Shin, T.; Shmeleva, A.; Shochet, M. J.; Short, D.; Shupe, M. A.; Sicho, P.; Sidoti, A.; Siebel, A.; Siegert, F.; Siegrist, J.; Sijacki, Dj.; Silbert, O.; Silva, J.; Silver, Y.; Silverstein, D.; Silverstein, S. B.; Simak, V.; Simard, O.; Simic, Lj.; Simion, S.; Simmons, B.; Simonyan, M.; Sinervo, P.; Sinev, N. B.; Sipica, V.; Siragusa, G.; Sisakyan, A. N.; Sivoklokov, S. Yu.; Sjölin, J.; Sjursen, T. B.; Skinnari, L. A.; Skovpen, K.; Skubic, P.; Skvorodnev, N.; Slater, M.; Slavicek, T.; Sliwa, K.; Sloan, T. J.; Sloper, J.; Smakhtin, V.; Smirnov, S. Yu.; Smirnova, L. N.; Smirnova, O.; Smith, B. C.; Smith, D.; Smith, K. M.; Smizanska, M.; Smolek, K.; Snesarev, A. A.; Snow, S. W.; Snow, J.; Snuverink, J.; Snyder, S.; Soares, M.; Sobie, R.; Sodomka, J.; Soffer, A.; Solans, C. A.; Solar, M.; Solc, J.; Soldatov, E.; Soldevila, U.; Solfaroli Camillocci, E.; Solodkov, A. A.; Solovyanov, O. V.; Sondericker, J.; Soni, N.; Sopko, V.; Sopko, B.; Sorbi, M.; Sosebee, M.; Soukharev, A.; Spagnolo, S.; Spanò, F.; Spighi, R.; Spigo, G.; Spila, F.; Spiriti, E.; Spiwoks, R.; Spousta, M.; Spreitzer, T.; Spurlock, B.; Denis, R. D. St.; Stahl, T.; Stahlman, J.; Stamen, R.; Stanecka, E.; Stanek, R. W.; Stanescu, C.; Stapnes, S.; Starchenko, E. A.; Stark, J.; Staroba, P.; Starovoitov, P.; Staude, A.; Stavina, P.; Stavropoulos, G.; Steele, G.; Steinbach, P.; Steinberg, P.; Stekl, I.; Stelzer, B.; Stelzer, H. J.; Stelzer-Chilton, O.; Stenzel, H.; Stevenson, K.; Stewart, G. A.; Stillings, J. A.; Stockmanns, T.; Stockton, M. C.; Stoerig, K.; Stoicea, G.; Stonjek, S.; Strachota, P.; Stradling, A. R.; Straessner, A.; Strandberg, J.; Strandberg, S.; Strandlie, A.; Strang, M.; Strauss, E.; Strauss, M.; Strizenec, P.; Ströhmer, R.; Strom, D. M.; Strong, J. A.; Stroynowski, R.; Strube, J.; Stugu, B.; Stumer, I.; Stupak, J.; Sturm, P.; Soh, D. A.; Su, D.; Subramania, H. S.; Succurro, A.; Sugaya, Y.; Sugimoto, T.; Suhr, C.; Suita, K.; Suk, M.; Sulin, V. V.; Sultansoy, S.; Sumida, T.; Sun, X.; Sundermann, J. E.; Suruliz, K.; Sushkov, S.; Susinno, G.; Sutton, M. R.; Suzuki, Y.; Svatos, M.; Sviridov, Yu. M.; Swedish, S.; Sykora, I.; Sykora, T.; Szeless, B.; Sánchez, J.; Ta, D.; Tackmann, K.; Taffard, A.; Tafirout, R.; Taga, A.; Taiblum, N.; Takahashi, Y.; Takai, H.; Takashima, R.; Takeda, H.; Takeshita, T.; Talby, M.; Talyshev, A.; Tamsett, M. C.; Tanaka, J.; Tanaka, R.; Tanaka, S.; Tanaka, S.; Tanaka, Y.; Tani, K.; Tannoury, N.; Tappern, G. P.; Tapprogge, S.; Tardif, D.; Tarem, S.; Tarrade, F.; Tartarelli, G. F.; Tas, P.; Tasevsky, M.; Tassi, E.; Tatarkhanov, M.; Taylor, C.; Taylor, F. E.; Taylor, G. N.; Taylor, W.; Castanheira, M. Teixeira Dias; Teixeira-Dias, P.; Temming, K. K.; Ten Kate, H.; Teng, P. K.; Terada, S.; Terashi, K.; Terron, J.; Terwort, M.; Testa, M.; Teuscher, R. J.; Thadome, J.; Therhaag, J.; Theveneaux-Pelzer, T.; Thioye, M.; Thoma, S.; Thomas, J. P.; Thompson, E. N.; Thompson, P. D.; Thompson, P. D.; Thompson, A. S.; Thomson, E.; Thomson, M.; Thun, R. P.; Tic, T.; Tikhomirov, V. O.; Tikhonov, Y. A.; Timmermans, C. J. W. P.; Tipton, P.; Tique Aires Viegas, F. J.; Tisserant, S.; Tobias, J.; Toczek, B.; Todorov, T.; Todorova-Nova, S.; Toggerson, B.; Tojo, J.; Tokár, S.; Tokunaga, K.; Tokushuku, K.; Tollefson, K.; Tomoto, M.; Tompkins, L.; Toms, K.; Tong, G.; Tonoyan, A.; Topfel, C.; Topilin, N. D.; Torchiani, I.; Torrence, E.; Torres, H.; Torró Pastor, E.; Toth, J.; Touchard, F.; Tovey, D. R.; Traynor, D.; Trefzger, T.; Tremblet, L.; Tricoli, A.; Trigger, I. M.; Trincaz-Duvoid, S.; Trinh, T. N.; Tripiana, M. F.; Trischuk, W.; Trivedi, A.; Trocmé, B.; Troncon, C.; Trottier-McDonald, M.; Trzupek, A.; Tsarouchas, C.; Tseng, J. C.-L.; Tsiakiris, M.; Tsiareshka, P. V.; Tsionou, D.; Tsipolitis, G.; Tsiskaridze, V.; Tskhadadze, E. G.; Tsukerman, I. I.; Tsulaia, V.; Tsung, J.-W.; Tsuno, S.; Tsybychev, D.; Tua, A.; Tuggle, J. M.; Turala, M.; Turecek, D.; Turk Cakir, I.; Turlay, E.; Turra, R.; Tuts, P. M.; Tykhonov, A.; Tylmad, M.; Tyndel, M.; Tyrvainen, H.; Tzanakos, G.; Uchida, K.; Ueda, I.; Ueno, R.; Ugland, M.; Uhlenbrock, M.; Uhrmacher, M.; Ukegawa, F.; Unal, G.; Underwood, D. G.; Undrus, A.; Unel, G.; Unno, Y.; Urbaniec, D.; Urkovsky, E.; Urrejola, P.; Usai, G.; Uslenghi, M.; Vacavant, L.; Vacek, V.; Vachon, B.; Vahsen, S.; Valenta, J.; Valente, P.; Valentinetti, S.; Valkar, S.; Valladolid Gallego, E.; Vallecorsa, S.; Valls Ferrer, J. A.; van der Graaf, H.; van der Kraaij, E.; Van Der Leeuw, R.; van der Poel, E.; van der Ster, D.; Van Eijk, B.; van Eldik, N.; van Gemmeren, P.; van Kesteren, Z.; van Vulpen, I.; Vandelli, W.; Vandoni, G.; Vaniachine, A.; Vankov, P.; Vannucci, F.; Varela Rodriguez, F.; Vari, R.; Varnes, E. W.; Varouchas, D.; Vartapetian, A.; Varvell, K. E.; Vassilakopoulos, V. I.; Vazeille, F.; Vegni, G.; Veillet, J. J.; Vellidis, C.; Veloso, F.; Veness, R.; Veneziano, S.; Ventura, A.; Ventura, D.; Venturi, M.; Venturi, N.; Vercesi, V.; Verducci, M.; Verkerke, W.; Vermeulen, J. C.; Vest, A.; Vetterli, M. C.; Vichou, I.; Vickey, T.; Viehhauser, G. H. A.; Viel, S.; Villa, M.; Villaplana Perez, M.; Vilucchi, E.; Vincter, M. G.; Vinek, E.; Vinogradov, V. B.; Virchaux, M.; Virzi, J.; Vitells, O.; Viti, M.; Vivarelli, I.; Vives Vaque, F.; Vlachos, S.; Vlasak, M.; Vlasov, N.; Vogel, A.; Vokac, P.; Volpi, G.; Volpi, M.; Volpini, G.; von der Schmitt, H.; von Loeben, J.; von Radziewski, H.; von Toerne, E.; Vorobel, V.; Vorobiev, A. P.; Vorwerk, V.; Vos, M.; Voss, R.; Voss, T. T.; Vossebeld, J. H.; Vranjes, N.; Vranjes Milosavljevic, M.; Vrba, V.; Vreeswijk, M.; Anh, T. Vu; Vuillermet, R.; Vukotic, I.; Wagner, W.; Wagner, P.; Wahlen, H.; Wakabayashi, J.; Walbersloh, J.; Walch, S.; Walder, J.; Walker, R.; Walkowiak, W.; Wall, R.; Waller, P.; Wang, C.; Wang, H.; Wang, H.; Wang, J.; Wang, J.; Wang, J. C.; Wang, R.; Wang, S. M.; Warburton, A.; Ward, C. P.; Warsinsky, M.; Watkins, P. M.; Watson, A. T.; Watson, M. F.; Watts, G.; Watts, S.; Waugh, A. T.; Waugh, B. M.; Weber, J.; Weber, M.; Weber, M. S.; Weber, P.; Weidberg, A. R.; Weigell, P.; Weingarten, J.; Weiser, C.; Wellenstein, H.; Wells, P. S.; Wen, M.; Wenaus, T.; Wendler, S.; Weng, Z.; Wengler, T.; Wenig, S.; Wermes, N.; Werner, M.; Werner, P.; Werth, M.; Wessels, M.; Weydert, C.; Whalen, K.; Wheeler-Ellis, S. J.; Whitaker, S. P.; White, A.; White, M. J.; White, S.; Whitehead, S. R.; Whiteson, D.; Whittington, D.; Wicek, F.; Wicke, D.; Wickens, F. J.; Wiedenmann, W.; Wielers, M.; Wienemann, P.; Wiglesworth, C.; Wiik, L. A. M.; Wijeratne, P. A.; Wildauer, A.; Wildt, M. A.; Wilhelm, I.; Wilkens, H. G.; Will, J. Z.; Williams, E.; Williams, H. H.; Willis, W.; Willocq, S.; Wilson, J. A.; Wilson, M. G.; Wilson, A.; Wingerter-Seez, I.; Winkelmann, S.; Winklmeier, F.; Wittgen, M.; Wolter, M. W.; Wolters, H.; Wooden, G.; Wosiek, B. K.; Wotschack, J.; Woudstra, M. J.; Wraight, K.; Wright, C.; Wrona, B.; Wu, S. L.; Wu, X.; Wu, Y.; Wulf, E.; Wunstorf, R.; Wynne, B. M.; Xaplanteris, L.; Xella, S.; Xie, S.; Xie, Y.; Xu, C.; Xu, D.; Xu, G.; Yabsley, B.; Yamada, M.; Yamamoto, A.; Yamamoto, K.; Yamamoto, S.; Yamamura, T.; Yamaoka, J.; Yamazaki, T.; Yamazaki, Y.; Yan, Z.; Yang, H.; Yang, U. K.; Yang, Y.; Yang, Y.; Yang, Z.; Yanush, S.; Yao, W.-M.; Yao, Y.; Yasu, Y.; Ybeles Smit, G. V.; Ye, J.; Ye, S.; Yilmaz, M.; Yoosoofmiya, R.; Yorita, K.; Yoshida, R.; Young, C.; Youssef, S.; Yu, D.; Yu, J.; Yu, J.; Yuan, L.; Yurkewicz, A.; Zaets, V. G.; Zaidan, R.; Zaitsev, A. M.; Zajacova, Z.; Zalite, Yo. K.; Zanello, L.; Zarzhitsky, P.; Zaytsev, A.; Zeitnitz, C.; Zeller, M.; Zemla, A.; Zendler, C.; Zenin, A. V.; Zenin, O.; Ženiš, T.; Zenonos, Z.; Zenz, S.; Zerwas, D.; della Porta, G. Zevi; Zhan, Z.; Zhang, D.; Zhang, H.; Zhang, J.; Zhang, X.; Zhang, Z.; Zhao, L.; Zhao, T.; Zhao, Z.; Zhemchugov, A.; Zheng, S.; Zhong, J.; Zhou, B.; Zhou, N.; Zhou, Y.; Zhu, C. G.; Zhu, H.; Zhu, J.; Zhu, Y.; Zhuang, X.; Zhuravlov, V.; Zieminska, D.; Zimmermann, R.; Zimmermann, S.; Zimmermann, S.; Ziolkowski, M.; Zitoun, R.; Živković, L.; Zmouchko, V. V.; Zobernig, G.; Zoccoli, A.; Zolnierowski, Y.; Zsenei, A.; zur Nedden, M.; Zutshi, V.; Zwalinski, L.

2011-10-01

Making use of 36 pb-1 of proton-proton collision data at sqrt{s} =7 TeV, the ATLAS Collaboration has performed a search for diphoton events with large missing transverse energy. Observing no excess of events above the Standard Model prediction, a 95% Confidence Level (CL) upper limit is set on the cross section for new physics of σ<0.38-0.65 pb in the context of a generalised model of gauge-mediated supersymmetry breaking (GGM) with a bino-like lightest neutralino, and of σ<0.18-0.23 pb in the context of a specific model with one universal extra dimension (UED). A 95% CL lower limit of 560 GeV, for bino masses above 50 GeV, is set on the GGM gluino mass, while a lower limit of 1/ R>961 GeV is set on the UED compactification radius R. These limits provide the most stringent tests of these models to date.

Th-U-total Pb Geochronology of Authigenic Monazite Near the top of the Sturtian-Marinoan Interglacial, Adelaide Rift Complex, South Australia

NASA Astrophysics Data System (ADS)

Mahan, K. H.; Wernicke, B. P.; Jercinovic, M. J.

2007-12-01

The Adelaide Rift Complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemostratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses ("snowball Earth") and Rodinia supercontinent reconstructions, but precise ages on igneous units do not yet exist. We report preliminary results of in situ Th-U-total Pb electron microprobe dating of monazite in sandstones within the Holowilena Ironstone ("older" Sturtian glacial at Enorama Creek) and at the top of the Enorama Shale (youngest pre-Marinoan, interglacial clastics at Elatina Creek). Several distinct populations are recognized. First, rounded cores with high Th, U, and Y + HREE abundances are interpreted as igneous or metamorphic detrital grains and yield ca. 1590 Ma, ca. 1280-1300 Ma, and ca. 1040 Ma dates related to well-known orogenic events in surrounding cratonic regions. A second group also occurs as "cores" but contains significantly lower U and Y + HREE, characteristics that may be indicative of an authigenic origin. Some rounded domains may represent "recycled" authigenic grains and yield dates of ca. 880 Ma and ca. 760 Ma. However, a subset observed in the Enorama sample occurs as very small (~2 x 10 microns), euhedral lathes that are unlikely to have survived a detrital history and yield a date of 680 +/-23 Ma. The youngest population forms very low Th and U, inclusion-rich overgrowths with ca. 500 Ma dates (Delamerian Orogeny) that probably grew hydrothermally. The recognition of "recycled" authigenic monazite further emphasizes the detail in textural and petrological documentation that is required for accurate geochronological interpretations. The date of 680 +/-23 Ma (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) is confirming of proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China.

Dating of barite and anhydrite in sea-floor hydrothermal deposits in the Okinawa Trough

NASA Astrophysics Data System (ADS)

Taisei, F.; Toyoda, S.; Uchida, A.; Ishibashi, J. I.; Totsuka, S.; Shimada, K.; Nakai, S.

2016-12-01

Dating of submarine hydrothermal activities has been an important issue in the aspect of the ore formation (Urabe, 1995) and biological systems sustained by the chemical species arising from hydrothermal activities (Macdonald et al., 1980). For this purpose, dating methods using radioactive disequilibrium such as U-Th method (e.g. You and Bickle, 1998) for sulfide, 226Ra-210Pb and 228Ra-228Th (e.g. Noguchi et al., 2011), Ra/Ba, and ESR (Electron Spin Resonance) methods for barite (Okumura et al., 2010) have been employed. In this study, firstly, we will report the first successful dating results on anhydrite using 226Ra-210Pb and 228Ra-228Th methods. The anhydrite samples were taken from the Daiyon-Yonaguni knoll field and the Hatoma knoll field and the Iheya North Knoll field of the Okinawa Trough by research cruises operated by JAMSTEC. The anhydrite crystals were physically scratched out of the samples. 226Ra, 228Ra and daughter nuclei were measured in the same samples for the ESR method by the low background gamma ray spectrometry. From the activity ratios, disequilibrium ages were obtained to be about 7.3 years by 226Ra-210Pb method, and to be 0.6-2.5 years by 228Ra-228Th method. Secondly, the ESR ages of barite taken from hydrothermal areas in the Okinawa trough range from 4.1 to 16000 years, filling the age gap of the maximum age, 150 years, of 226Ra-210Pb method and the minimum age, several thousand years of U-Th method, being the most appropriate age range to discuss the evolution of the hydrothermal systems. Interestingly, the 226Ra-210Pb and 228Ra-228Th ages for the same samples are the same or younger than the ESR ages. As for the latter samples, the reason has already been discussed (Uchida et al., 2015) as the deposits had been formed by two or more hydrothermal events. In the present paper, the disequilibrium and ESR ages will be simulated with these multiple hydrothermal events so that the differences in the ages are explained.

High-precision ID-TIMS zircon U-Pb geochronology using new 1013 Ohm resistors

NASA Astrophysics Data System (ADS)

Von Quadt, A.; Buret, Y.; Large, S.; Peytcheva, I.; Trinquier, A.; Wotzlaw, J. F.

2015-12-01

Faraday cups equipped with high gain amplifiers provide a means to measure small ion beams in static mode without the limited linear range of ion counting systems. We tested the application of newly available 1013 Ohm resistors to ID-TIMS zircon U-Pb geochronology using a range of natural and synthetic reference materials. The TritonPlus-RPQ at the Institute of Geochemistry and Petrology, ETH Zurich, is equipped with five new 1013 Ohm resistors and one MasCom secondary electron multiplier, allowing to measure the 202-204-205-206-207-208Pb masses in static mode. U is measured subsequently as U-oxide (265-267-270UO2) during a second step, also in static Faraday mode. The gain calibration of the 1013 Ohm resistors was performed using the procedure of Trinquier (2014), with 144Nd-146Nd being measured using 1011 Ohm resistor and 142-143-145-148-150Nd being measured using 1013 Ohm resitors (Trinquier, 2014; Koornneef et al., 2014). Standard deviations of the noise in all five new 1013 Ohm resistors are lower than 5.0 x 10-6 over a 6 month period, with no shift occurring over this time interval. This new detector set-up was tested by analyzing natural zircon standard materials and synthetic U/Pb solutions (www.earthime.org), ranging in age from ~2 Ma to ~600 Ma. All natural zircon standards were chemically abraded (Mattinson, 2005) and all samples were spiked with the ET2535 tracer solution. U-Pb dates obtained using the static measurement routine are compared to measurements employing dynamic peak jumping routines on the MasCom multiplier. This study illustrates the benefits and current limitations of using high gain amplifiers to measure small ion beams for zircon U-Pb geochronology compared to conventional dynamic ion counting techniques. Mattinson, J.M. (2005) Chemical Geology 220:47-66; Trinquier, A. (2014) Application Note 30281; Koornneef, J. et al (2014) Analytica Chimica Acta 819:49-55.

Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta

NASA Technical Reports Server (NTRS)

Krogh, T. E.; Kamo, S. L.; Bohor, B. F.

1993-01-01

U-Pb isotopic dating of single 1 - 3 micrograms zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a component of the target site, while those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

Evidence for prolonged mid-Paleozoic plutonism and ages of crustal sources in east-central Alaska from SHRIMP U-Pb dating of syn-magmatic, inherited, and detrital zircon

USGS Publications Warehouse

Dusel-Bacon, C.; Williams, I.S.

2009-01-01

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb analyses of igneous zircons from the Lake George assemblage in the eastern Yukon-Tanana Upland (Tanacross quadrangle) indicate both Late Devonian (???370 Ma) and Early Mississippian (???350 Ma) magmatic pulses. The zircons occur in four textural variants of granitic orthogneiss from a large area of muscovite-biotite augen gneiss. Granitic orthogneiss from the nearby Fiftymile batholith, which straddles the Alaska-Yukon border, yielded a similar range in zircon U-Pb ages, suggesting that both the Fiftymile batholith and the Tanacross orthogneiss body consist of multiple intrusions. We interpret the overall tectonic setting for the Late Devonian and Early Mississippian magmatism as an extending continental margin (broad back-arc region) inboard of a northeast-dipping (present coordinates) subduction zone. New SHRIMP U-Pb ages of inherited zircon cores in the Tanacross orthogneisses and of detrital zircons from quartzite from the Jarvis belt in the Alaska Range (Mount Hayes quadrangle) include major 2.0-1.7 Ga clusters and lesser 2.7-2.3 Ga clusters, with subordinate 3.2, 1.4, and 1.1 Ga clusters in some orthogneiss samples. For the most part, these inherited and core U-Pb ages match those of basement provinces of the western Canadian Shield and indicate widespread potential sources within western Laurentia for most grain populations; these ages also match the detrital zircon reference for the northern North American miogeocline and support a correlation between the two areas.

Primary Data on U/Pb-Isotope Ages and Lu/Hf-Isotope Geochemical Systematization of Detrital Zircons from the Lopatinskii Formation (Vendian-Cambrian Transition Levels) and the Tectonic Nature of Teya-Chapa Depression (Northeastern Yenisei Ridge)

NASA Astrophysics Data System (ADS)

Kuznetsov, N. B.; Priyatkina, N. S.; Rud'ko, S. V.; Shatsillo, A. V.; Collins, W. J.; Romanyuk, T. V.

2018-03-01

The main results are presented on U/Pb-isotope dating of 100 detrital zircons and, selectively, on the Lu/Hf-isotope system of 43 grains from sandstones of the Lopatinskii formation (the lower stratigraphic level of the Chingasan group). Ages from 896 ± 51 to 2925 ± 38 Ma were obtained with a pronounced maximum of 1890 Ma in the curve of probability density, along with ɛHf estimates from +8.4 to-15.1, which allow one to throw doubt upon the molasse nature of the Lopatinskii formation.

Analysis of a Uranium Oxide Sample Interdicted in Slovakia (FSC 12-3-1)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Borg, Lars E.; Dai, Zurong; Eppich, Gary R.

2014-01-17

We provide a concise summary of analyses of a natural uranium sample seized in Slovakia in November 2007. Results are presented for compound identification, water content, U assay, trace element abundances, trace organic compounds, isotope compositions for U, Pb, Sr and O, and age determination using the 234U – 230Th and 235U – 231Pa chronometers. The sample is a mixture of two common uranium compounds - schoepite and uraninite. The uranium isotope composition is indistinguishable from natural; 236U was not detected. The O, Sr and Pb isotope compositions and trace element abundances are unremarkable. The 234U – 230Th chronometer givesmore » an age of 15.5 years relative to the date of analysis, indicating the sample was produced in January 1997. A comparison of the data for this sample with data in the Uranium Sourcing database failed to find a match, indicating the sample was not produced at a facility represented in the database.« less

The Age of the intra-Danubian Suture (Southern Carpathians, Romania)

NASA Astrophysics Data System (ADS)

Balica, C.; Hann, H. P.; Chen, F.; Balintoni, I. C.; Zaharia, L.

2007-12-01

The Southern Carpathians, as an Alpine chain are formed of two domains, namely the Getic Domain (GD) and the Danubian Domain (DD). The basement of DD is represented by two terranes, named Dragsan and Lainici- Paius sutured through Tisovita-Iuti ophiolitic complex. The two terranes were invaded by large granitic plutons, some of them being dated as Late Proterozoic by U/Pb method. Yet, along the inferred suture there are four granitic bodies whose ages have been only assumed by their geological relations. From North to South the four bodies sampled for LA-ICP-MS zircon U/Pb dating are: Muntele Mic, Sfardinu, Cherbelezu and Ogradena. The previously CL imaged zircon crystals were ablated at the China's University of Geosciences facilities in Wuhan. The zircons from all samples showed quite complex structures, with many inherited cores or affected by lead loss processes. In order to get a mean age for every pluton, we used the weighted average plots by projecting the 206Pb/238U apparent ages. The crystallization age of the Poiana Marului pluton is around 326.7±7 Ma (MSWD 1.6). A set of sixteen apparent ages ranging between 400 to 648 Ma together with other tree points indicating 897, 1353 and 1693 Ma, represent inheritances. There was no observable lead loss process in this data set. The Sfardinu granite crystallized at 310±7.9 Ma (MSWD 4.8). The inheritances found in this sample are at 427 and 723 Ma, but an important lead loss process occurred later, as indicated by ten apparent ages between 240-292 Ma. Cherbelezu granite gave a crystallization mean age of 326.9±4.9 Ma (MSWD 1.9). A single inherited core appears at 502 Ma, yet eight apparent ages ranging between 239-295 Ma signalize again an important lead loss process. Other two ages at 176 and 193 Ma confirm the presence of this process. The Ogradena pluton zircons display two possible crystallization ages for the outer zones of zircon grains, at 356.6±7.8 Ma (twelve apparent ages, MSWD 12) and 314.1±7.8 Ma (nine apparent ages, MSWD 13) respectively. It is difficult to explain this situation, and we suppose the second figure as the probable crystallization age. Inheritances are represented by a number of 37 206Pb/238U apparent ages scattered between 400 and 612 Ma, only one apparent age of 265 Ma possibly proving the presence of lead loss. The geochemical parameters ASI, Fe-number and MALI calculated from the major oxides analyses showed that all four plutons are metaluminous and magnezian. Both Cherbelezu and Ogradena have a calcalkaline character near to alkalicalcic while Muntele Mic and Sfardinu are alkalicalcic. Geochemically, all four plutons are closed to Cordilleran type, main portion or a little inboard of it. Out of these data several conclusions can be drawn. The age of the intra-Danubian suture is late Visean to Bashkirian or late Variscan. Muntele Mic and Ogradena granites exhibit abundant Cadomian inheritances, while Cherbelezu and Sfardinu plutons seem affected by lead loss processes due to an important thermotectonic event happened during 250 to 290 Ma interval. The emplacement of the four plutons probably reflects a subductional process. Finally, the inheritances suggest a Gondwanan source for the anatectic material.

New dating evidence of the early presence of hominins in Southern Europe.

PubMed

Michel, Véronique; Shen, Chuan-Chou; Woodhead, Jon; Hu, Hsun-Ming; Wu, Chung-Che; Moullé, Pierre-Élie; Khatib, Samir; Cauche, Dominique; Moncel, Marie-Hélène; Valensi, Patricia; Chou, Yu-Min; Gallet, Sylvain; Echassoux, Anna; Orange, François; de Lumley, Henry

2017-08-30

The first "Out of Africa" migrations represent a seminal event in the history of humankind. At the gates of Europe, the first appearance of Hominins is recorded in Georgia, 1.8 million years ago (Ma); however, the picture of migration across the continent remains incomplete. Vallonnet Cave (France) is a Lower Paleolithic prehistoric site with traces of hominin activities including lithic remains and cut-marks on mammal bones. Here, we apply the uranium-lead (U-Pb) methods to two flowstones to date the intervening archaeological levels. The U-Pb data, coupled with paleomagnetic constraints, provide an age range from 1.2 to 1.1 Ma. The results conclusively demonstrate that Vallonnet Cave is one of the oldest European prehistoric sites in France with early hominin occupations associated with an Epivillafranchian fauna. Combined with data from other archaeological sites, the new precise chronology suggests a widespread occupation the Northern Mediterranean to Southwestern Europe at ~1.2 Ma.

Permian A-type rhyolites of the Muráň Nappe, Inner Western Carpathians, Slovakia: in-situ zircon U-Pb SIMS ages and tectonic setting

NASA Astrophysics Data System (ADS)

Ondrejka, Martin; Li, Xian-Hua; Vojtko, Rastislav; Putis, Marian; Uher, Pavel; Sobocký, Tomas

2018-04-01

Three representative A-type rhyolitic rock samples from the Muráň Nappe of the inferred Silicic Unit of the Inner Western Carpathians (Slovakia) were dated using the high-precision SIMS U-Pb isotope technique on zircons. The geochronological data presented in this paper is the first in-situ isotopic dating of these volcanic rocks. Oscillatory zoned zircon crystals mostly revealed concordant Permian (Guadalupian) ages: 266.6 ± 2.4 Ma in Tisovec-Rejkovo (TIS-1), 263.3 ± 1.9 Ma in Telgárt-Gregová Hill (TEL-1) and 269.5 ± 1.8 Ma in Veľká Stožka-Dudlavka (SD-2) rhyolites. The results indicate that the formation of A-type rhyolites and their plutonic equivalents are connected to magmatic activity during the Permian extensional tectonics and most likely related to the Pangea supercontinent break-up.

Geochronology, petrogenesis and tectonic settings of pre- and syn-ore granites from the W-Mo deposits (East Kounrad, Zhanet and Akshatau), Central Kazakhstan

NASA Astrophysics Data System (ADS)

Li, GuangMing; Cao, MingJian; Qin, KeZhang; Evans, Noreen J.; Hollings, Pete; Seitmuratova, Eleonora Yusupovha

2016-05-01

There is significant debate regarding the mineralization ages of the East Kounrad, Zhanet and Akshatau W-Mo deposits of Central Kazakhstan, and the petrogenesis and tectono-magmatic evolution of the granites associated with these deposits. To address these issues, we present molybdenite Re-Os dating, zircon U-Pb dating, whole rock geochemistry as well as Sr-Nd-Pb and zircon O-Hf isotopic analyses on the pre-mineralization and ore-forming granites. U-Pb dating of zircons from pre-mineralization granitic rocks yield Late Carboniferous ages of 320-309 Ma, whereas ore-forming granites have Early Permian ages of 298-285 Ma. Molybdenite Re-Os isotopic data indicate a mineralization age of 296 Ma at East Kounrad, 294 Ma at Akshatau and 285 Ma at Zhanet. The pre-ore and ore-forming granites are high-K calc-alkaline, metaluminous to slightly peraluminous I-type granites. The pre-mineralization granites are relatively unfractionated, whereas the ore-forming granites are highly fractionated. The fractionating mineral phases are probably K-feldspar, apatite, Ti-bearing phases and minor plagioclase. The pre-mineralization and ore-forming rocks are characterized by similar Sr-Nd-Pb-Hf-O isotopic compositions ((87Sr/86Sr)i = 0.70308-0.70501, εNd (t) = - 0.5 to + 2.8, 207Pb/204Pb = 15.60-15.82, zircon εHf (t) = + 1.2 to + 15.6 and δ18O = + 4.6 to + 10.3‰), whole rock TDMC (Nd) (840-1120 Ma) and zircon TDMC (Hf) (320-1240 Ma). The isotopic characteristics are consistent with a hybrid magma source caused by 10-30% assimilation of ancient crust by juvenile lower crust. The geochronology and geochemistry of these granites show that the Late Carboniferous pre-mineralization granitic rocks formed during subduction, whereas the Early Permian ore-forming, highly fractionated granite probably underwent significant fractionation with a restite assemblage of K-feldspar, apatite, Ti-bearing phases and minor plagioclase and developed during collision between the Yili and Kazakhstan terranes commenced at the latest Late Carboniferous.

Zircon age-temperature-compositional spectra in plutonic rocks

DOE PAGES

Samperton, Kyle M.; Bell, Elizabeth A.; Barboni, Mélanie; ...

2017-08-23

We present that geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (T sat) and rock solidification (T solid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at T sat decreasing continuously to volumetrically minor young, cold crystallization at T solid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional changemore » over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. In conclusion, these findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record.« less

Zircon age-temperature-compositional spectra in plutonic rocks

DOE Office of Scientific and Technical Information (OSTI.GOV)

Samperton, Kyle M.; Bell, Elizabeth A.; Barboni, Mélanie

We present that geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (T sat) and rock solidification (T solid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at T sat decreasing continuously to volumetrically minor young, cold crystallization at T solid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional changemore » over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. In conclusion, these findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record.« less

Zircon U-Pb geochronology and Sr-Nd-Pb-Hf isotopic constraints on the timing and origin of Mesozoic granitoids hosting the Mo deposits in northern Xilamulun district, NE China

NASA Astrophysics Data System (ADS)

Shu, Qihai; Lai, Yong; Zhou, Yitao; Xu, Jiajia; Wu, Huaying

2015-12-01

Located in the east section of the Central Asian orogen in northeastern China, the Xilamulun district comprises several newly discovered molybdenum deposits, primarily of porphyry type and Mesozoic ages. This district is divided by the Xilamulun fault into the southern and the northern parts. In this paper, we present new zircon U-Pb dating, trace elements and Hf isotope, and/or whole rock Sr-Nd-Pb isotopic results for the host granitoids from three Mo deposits (Yangchang, Haisugou and Shabutai) in northern Xilamulun. Our aim is to constrain the age and petrogenesis of these intrusions and their implications for Mo mineralization. Zircon U-Pb LA-ICP-MS dating shows that the monzogranites from the Shabutai and Yangchang deposits formed at 138.4 ± 1.5 and 137.4 ± 2.1 Ma, respectively, which is identical to the molybdenite Re-Os ages and coeval well with the other Mo deposits in this region, thereby indicating an Early Cretaceous magmatism and Mo mineralization event. Zircon Ce/Nd ratios from the mineralized intrusions are significantly higher than the barren granites, implying that the mineralization-related magmas are characterized by higher oxygen fugacity. These mineralized intrusions share similar zircon in-situ Hf and whole rock Sr-Nd isotopic compositions, with slightly negative to positive εHf(t) ranging from - 0.8 to + 10.0, restricted εNd(t) values from - 3.7 to + 1.6 but a little variable (87Sr/86Sr)i ratios between 0.7021 and 0.7074, indicative of formation from primary magmas generated from a dominantly juvenile lower crust source derived from depleted mantle, despite diverse consequent processes (e.g., magma mixing, fractional crystallization and crustal contamination) during their evolution. The Pb isotopes (whole rock) also show a narrow range of initial compositions, with (206Pb/204Pb)i = 18.03-18.88, (207Pb/204Pb)i = 15.48-15.58 and (208Pb/204Pb)i = 37.72-38.28, in agreement with Sr-Nd-Hf isotopes reflecting the dominance of a mantle component. An integration of geochronological and isotopic data points to three stages of Mo mineralization events (i.e., Triassic, Late Jurassic, and Early Cretaceous) associated with magmas generated from three different source regions in the Xilamulun district, NE China. The variation in the origin of the magmas from which the porphyry Mo systems were generated suggests that the formation of Mo deposit lies not in the composition of magma sources. Other factors, including high magma oxygen fugacity, may have taken a fundamental role in Mo enrichment and subsequent mineralization.

Simultaneous in situ determination of both U-Th-Pb and Sm-Nd isotopes in monazite by laser ablation using a magnetic sector ICP-MS and a multicollector ICP-MS

NASA Astrophysics Data System (ADS)

Goudie, D. J.; Fisher, C. M.; Hanchar, J. M.; Davis, W. J.; Crowley, J. L.; Ayers, J. C.

2012-12-01

We present a method for the simultaneous in situ determination of U-Th-Pb and Sm-Nd isotopes in monazite, using a laser ablation (LA) system coupled to both a magnetic sector inductively coupled plasma mass spectrometer (HR) ICP-MS and a multicollector (MC) ICP-MS. The ablated material is split using a glass Y-connector and transported simultaneously to both mass spectrometers via helium carrier gas. The MC-ICP-MS is configured to provide relative Ce, Gd, and Eu contents, in addition to Sm and Nd. This approach obtains both age (U-Pb), tracer isotope (Sm-Nd), and REE element data (Ce, Gd, and Eu), in the same ablation volume, thus reducing sampling problems associated with fine-scale zoning and other internal structures. The accuracy and precision of the U-Pb data are demonstrated using six well characterized monazite reference materials from the Geological Survey of Canada (three of which are currently used as SHRIMP standards) and agree well with previously determined ID-TIMS ages. The accuracy of the Sm-Nd isotopic data was assessed by comparison to TIMS measurements on a well-characterized in-house monazite standard. The dual LA-ICP-MS method was applied to the Birch Creek Pluton (BCP) in the White Mountains, California in a case study to test the utility of U-Th-Pb dating coupled with Sm-Nd (and Ce, Gd, Eu) isotopic data for solving geologic problems. Previous work on the Cretaceous BCP [1] used Th-Pb ages coupled with O isotopic data to constrain hydrothermal fluid events, as recorded in monazite. The original study suggested that the high delta 18O monazite in Paleozoic country rocks adjacent to the BCP grew in response to fluid alternation associated with the intrusion of the BCP, based on overlapping age with the BCP. New monazite split-stream U-Pb and Sm-Nd data show that monazite from the BCP pluton and monazite from altered country rock have homogenous and overlapping initial Nd isotopic composition, further strengthening the proposal that monazite in altered country rock can be a tracer of fluid alternation events. The split-stream U-Pb ages agrees with new high precision ID-TIMS U-Pb ages from the same monazite grains. These results demonstrate how monazite age and Sm-Nd isotopic data, coupled with delta 18O, can identify hydrothermal monazite and constrain the timing and potential sources of fluid events. [1] Ayers et al., Geology 34 (2006) 653-656.

U-series dating of impure carbonates: An isochron technique using total-sample dissolution

USGS Publications Warehouse

Bischoff, J.L.; Fitzpatrick, J.A.

1991-01-01

U-series dating is a well-established technique for age determination of Late Quaternary carbonates. Materials of sufficient purity for nominal dating, however, are not as common as materials with mechanically inseparable aluminosilicate detritus. Detritus contaminates the sample with extraneous Th. We propose that correction for contamination is best accomplished with the isochron technique using total sample dissolution (TSD). Experiments were conducted on artificial mixtures of natural detritus and carbonate and on an impure carbonate of known age. Results show that significant and unpredictable transfer of radionuclides occur from the detritus to the leachate in commonly used selective leaching procedures. The effects of correcting via leachate-residue pairs and isochron plots were assessed. Isochrons using TSD gave best results, followed by isochron plots of leachates only. ?? 1991.

Elemental and Molecular Segregation in Oil Paintings due to Lead Soap Degradation

DOE PAGES

Chen-Wiegart, Yu-chen Karen; Catalano, Jaclyn; Williams, Garth J.; ...

2017-09-14

The formation of Pb, Zn, and Cu carboxylates (soaps) has caused visible deterioration in hundreds of oil paintings dating from the 15th century to the present. Through transport phenomena not yet understood, free fatty acids in the oil binding medium migrate through the paint and react with pigments containing heavy metals to form soaps. To investigate the complex correlation among the elemental segregation, types of chemical compounds formed, and possible mechanisms of the reactions, a paint sample cross-section from a 15th century oil painting was examined by synchrotron X-ray techniques. X-ray fluorescence (XRF) microscopy, quantified with elemental correlation density distribution,more » showed Pb and Sn segregation in the soap-affected areas. X-ray absorption near edge structure (XANES) around the Pb-L3 absorption edge showed that Pb pigments and Pb soaps can be distinguished while micro-XANES gave further information on the chemical heterogeneity in the paint film. The advantages and limitations of these synchrotron-based techniques are discussed and compared to those of methods routinely used to analyze paint samples. The results presented set the stage for improving the information extracted from samples removed from works of art and for correlating observations in model paint samples to those in the naturally aged samples, to shed light onto the mechanism of soap formation.« less

Elemental and Molecular Segregation in Oil Paintings due to Lead Soap Degradation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen-Wiegart, Yu-chen Karen; Catalano, Jaclyn; Williams, Garth J.

The formation of Pb, Zn, and Cu carboxylates (soaps) has caused visible deterioration in hundreds of oil paintings dating from the 15th century to the present. Through transport phenomena not yet understood, free fatty acids in the oil binding medium migrate through the paint and react with pigments containing heavy metals to form soaps. To investigate the complex correlation among the elemental segregation, types of chemical compounds formed, and possible mechanisms of the reactions, a paint sample cross-section from a 15th century oil painting was examined by synchrotron X-ray techniques. X-ray fluorescence (XRF) microscopy, quantified with elemental correlation density distribution,more » showed Pb and Sn segregation in the soap-affected areas. X-ray absorption near edge structure (XANES) around the Pb-L3 absorption edge showed that Pb pigments and Pb soaps can be distinguished while micro-XANES gave further information on the chemical heterogeneity in the paint film. The advantages and limitations of these synchrotron-based techniques are discussed and compared to those of methods routinely used to analyze paint samples. The results presented set the stage for improving the information extracted from samples removed from works of art and for correlating observations in model paint samples to those in the naturally aged samples, to shed light onto the mechanism of soap formation.« less

Cretaceous joints in southeastern Canada: dating calcite-filled fractures

NASA Astrophysics Data System (ADS)

Schneider, David; Spalding, Jennifer; Gautheron, Cécile; Sarda, Philippe; Davis, Donald; Petts, Duane

2017-04-01

To resolve the timing of brittle tectonism is a challenge since the classical chronometers required for analyses are not often in equilibrium with the surrounding material or simply absent. In this study, we propose to couple LA-ICP-MS U-Pb and (U-Th)/He dating with geochemical proxies in vein calcite to tackle this dilemma. We examined intracratonic Middle Ordovician limestone bedrock that overlies Mesoproterozoic crystalline basement, which are cut by NE-trending fault zones that have historic M4-5 earthquakes along their trace. E-W to NE-SW vertical joint sets, the relatively youngest stress recorded in the bedrock, possess 1-7 mm thick calcite veins that seal fractures or coat fracture surfaces. The veins possess intragranular calcite that are lined with fine-grained calcite along the vein margin and can exhibit µm- to mm-scale offset (e.g. displaced fossil fragments in host rock). Calcite d18O and d13C values are analogous to the bulk composition of Middle to Late Ordovician limestones, and suggest vein formation from a source dominated by connate fluids. The calcite contain trails of fluid inclusions commonly along fractures, and 3He/4He analyses indicate a primitive, deep fluid signature (R/Ra: 0.5-2.7). Trace element geochemistry of the calcite is highly variable, generally following the elevated HREE and lower LREE of continental crust trends but individual crystals from a single vein may vary by three orders of magnitude. LA-ICP-MS geochemical traverse across veins show elevated concentrations along (sub)grain boundaries and the vein-host rock contact. Despite abundant helium concentrations, (U-Th)/He dating was unsuccessful yielding highly dispersed dates likely from excess helium derived from the fluid inclusions. However, LA-ICP-MS U-Pb dating on calcite separated from the veins yielded model ages of 110.7 ± 6.8 Ma (MSWD: 0.53; n: 16) to 81.4 ± 8.3 Ma (MSWD: 2.6; n: 17). Since all veins are from the same ENE-trend, we regressed all the calcite dates together, yielding an age of 101 ± 6 Ma (MSWD: 2.3). These veins are 200 km to the west and slightly younger than the c. 140-120 Ma alkaline igneous rocks which mark the surface trace of the Great Meteor Hotspot. The period of 110-90 Ma has been identified as a time of major plate reorganization that involved tectonic and magmatic events, which may be reflected in our new calcite dates. Nonetheless, LA-ICP-MS U-Pb dating of vein calcite was successful, and coupled with other geochemical information, can yield primary information about the timing and source of fluid flow through joints and fractures, which has direct applications to reducing risk associated with characterizing hydrocarbon reservoirs and deep geological repositories for nuclear waste.

In-Situ U–Pb Dating of Apatite by Hiroshima-SHRIMP: Contributions to Earth and Planetary Science

PubMed Central

Terada, Kentaro; Sano, Yuji

2012-01-01

The Sensitive High Resolution Ion MicroProbe (SHRIMP) is the first ion microprobe dedicated to geological isotopic analyses, especially in-situ analyses related to the geochronology of zircon. Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda (1974), was constructed at the Australian National University. In 1996, such an instrument was installed at Hiroshima University and was the first SHRIMP to be installed in Japan. Since its installation, our focus has been on the in-situ U–Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. This provides the possibility for extending the use of in-situ U–Pb dating from determining the age of formation of volcanic, granitic, sedimentary and metamorphic minerals to the direct determination of the diagenetic age of fossils and/or the crystallization age of various meteorites, which can provide new insights into the thermal history on the Earth and/or the Solar System. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years. PMID:24349912

Molybdenite Re-Os, zircon U-Pb dating and Lu-Hf isotopic analysis of the Xiaerchulu Au deposit, Inner Mongolia Province, China

NASA Astrophysics Data System (ADS)

Wang, Jia-xin; Nie, Feng-Jun; Zhang, Xue-ni; Jiang, Si-hong

2016-09-01

The Xiaerchulu Au deposit, located in the Southern Orogenic Belt (SOB) of Western Inner Mongolia (WIM), is hosted in an Early Permian (271-261 Ma) volcanic-plutonic sequence. Mineralization took place in silicified biotite granites or along the contact zone between the Neoproterozoic Baiyinbaolage Group and the biotite granite. In order to constrain the timing of the Xiaerchulu mineralization and discuss the petrogenesis of the hosting granites, molybdenite Re-Os, and zircon U-Pb and, Lu-Hf, and REE, geochemical, and Sr-Nd isotopic studies were completed in this study. We measured Re-Os isotopes of six molybdenite samples from the main ore body, which yielded a weighted average model age of 261.7 ± 1.5 Ma with a MSWD of 0.55, indicating that the time of mineralization was at ca. 262 Ma. High precision U-Pb dating for the studied granites yields Permian 206Pb/238U ages ranging from 271 to 269 Ma. These age data confirm that both the intrusion and related mineralization were initiated in Early Permian period. These granites are strongly peraluminous with A/CNK = 1.11-1.12, high SiO2-K2O contents, as well as containing biotite and muscovite, indicating a petrogenesis of typical S-type granites, the above consideration is also consistent with the result of discrimination diagrams. The Re contents of molybdenite, εNd(t), and zircon εHf(t), as well as the 176Hf/177Hf values of the granites, fall into the ranges from 1.153 to 2.740 μg/g, - 11.1 to - 9.3, - 8.8 to - 0.9, and 0.282358 to 0.282688, respectively. All of this evidence suggests that the metals were derived from a predominantly crustal source, the granites originated from crust in an extensional setting, and the rejuvenation of the continent may have play an important role during the ore-forming processes of the Early Permian epoch.

High-Resolution Zircon U-Pb CA-TIMS Dating of the Carboniferous—Permian Successions, Paraná Basin, Brazil

NASA Astrophysics Data System (ADS)

Griffis, N. P.; Mundil, R.; Montanez, I. P.; Isbell, J.; Fedorchuk, N.; Lopes, R.; Vesely, F.; Iannuzzi, R.

2015-12-01

The late Paleozoic Ice Age (LPIA) is Earth's only record of a CO2-forced climatic transition from an icehouse to greenhouse state in a vegetated world. Despite a refined framework of Gondwanan ice distribution, questions remain about the timing, volume, and synchronicity of high-latitude continental ice and the subsequent deglaciation. These questions ultimately preclude our understanding of linkages between ice volume, sea level, and high- and low-latitude climate. Poor constraints on the timing and synchronicity of glacial and interglacial transitions reflect a lack of high-resolution radioisotopic dates from high-latitude, ice-proximal Carboniferous-Permian successions. The Rio Bonito Fm in Rio Grande do Sul State of southern Brazil hosts the oldest non-glaciogenic Carboniferous- Permian deposits of the Paraná Basin, thus recording the icehouse-to-greenhouse transition. Despite a widespread effort over the last two decades to constrain these deposits in time by means of U-Pb zircon geochronology, published data sets of the Candiota and Faxinal coals of the Rio Bonito Fm host discrepancies that may reflect post- eruptive open system behavior of zircon and analytical artifacts. These discrepancies have hindered the correlation of the Candiota and Faxinal sediments within the larger Gondwanan framework. Here we present the first U-Pb ages on closed system single zircons using CA-TIMS techniques on Permo-Carboniferous ash deposits of the Paraná Basin. Preliminary results indicate two major and distinct coal-forming periods that are separated by ca 10 Ma. Our results and conclusions are not in agreement with multi- crystal U-Pb TIMS and SIMS ages that suggest coeval deposition of the Candiota and Faxinal coals. CA-TIMS analyses applied to zircons from additional ash deposits are aimed at constructing a robust chronostratigraphic framework for the Carboniferous- Permian succession of the Paraná Basin, which will facilitate a better understanding of the timing and ice dynamics of the LPIA.

U-Pb-Th geochronology of monazite and zircon in albitite metasomatites of the Rožňava-Nadabula ore field (Western Carpathians, Slovakia): implications for the origin of hydrothermal polymetallic siderite veins

NASA Astrophysics Data System (ADS)

Hurai, V.; Paquette, J.-L.; Lexa, O.; Konečný, P.; Dianiška, I.

2015-10-01

Sodic metasomatites (albitites) occur around and within siderite veins in the southern part of the Gemeric tectonic unit of the Western Carpathians. Accessory minerals of the metasomatites represented by monazite, zircon, apatite, rutile, tourmaline and siderite are basically identical with the quartz-tourmaline stage of other siderite and stibnite veins of the tectonic unit. Statistical analysis of chemical Th-U(total)-Pb isochron method (CHIME) of monazite dating yielded Jurassic-Cretaceous ages subdivided into 3-4 modes, spreading over time interval between 78 and 185 Ma. In contrast, LA-ICPMS 206Pb/238U dating carried out on the same monazite grains revealed a narrow crystallization interval, showing ages of Th-poor cores with phengite inclusions identical within the error limit with Th-rich rims with cauliflower-like structure. The determined lower intercept at 139 ± 1 Ma overlapped the Vallanginian-Berriasian boundary, thus corroborating the model of formation of hydrothermal vein structures within an arcuate deformation front built up in the Variscan basement as a response to Early Cretaceous compression, folding and thrusting. In contrast, associated zircons are considerably older than the surrounding Early-Palaeozoic volcano-sedimentary rocks, showing Neoproterozoic ages. The zircon grains in albitite metasomatites are thus interpreted as fragments of Pan-African magmatic detritus incorporated in the vein structures by buoyant hydrothermal fluids.

Algorithms and software for U-Pb geochronology by LA-ICPMS

NASA Astrophysics Data System (ADS)

McLean, Noah M.; Bowring, James F.; Gehrels, George

2016-07-01

The past 15 years have produced numerous innovations in geochronology, including experimental methods, instrumentation, and software that are revolutionizing the acquisition and application of geochronological data. For example, exciting advances are being driven by Laser-Ablation ICP Mass Spectrometry (LA-ICPMS), which allows for rapid determination of U-Th-Pb ages with 10s of micrometer-scale spatial resolution. This method has become the most commonly applied tool for dating zircons, constraining a host of geological problems. The LA-ICPMS community is now faced with archiving these data with associated analytical results and, more importantly, ensuring that data meet the highest standards for precision and accuracy and that interlaboratory biases are minimized. However, there is little consensus with regard to analytical strategies and data reduction protocols for LA-ICPMS geochronology. The result is systematic interlaboratory bias and both underestimation and overestimation of uncertainties on calculated dates that, in turn, decrease the value of data in repositories such as EarthChem, which archives data and analytical results from participating laboratories. We present free open-source software that implements new algorithms for evaluating and resolving many of these discrepancies. This solution is the result of a collaborative effort to extend the U-Pb_Redux software for the ID-TIMS community to the LA-ICPMS community. Now named ET_Redux, our new software automates the analytical and scientific workflows of data acquisition, statistical filtering, data analysis and interpretation, publication, community-based archiving, and the compilation and comparison of data from different laboratories to support collaborative science.

Pre-Variscan evolution of the Western Tatra Mountains: new insights from U-Pb zircon dating.

PubMed

Burda, Jolanta; Klötzli, Urs

In situ LA-MC-ICP-MS U-Pb zircon geochronology combined with cathodoluminescence imaging were carried out to determine protolith and metamorphic ages of orthogneisses from the Western Tatra Mountains (Central Western Carpathians). The metamorphic complex is subdivided into two units (the Lower Unit and the Upper Unit). Orthogneisses of the Lower Unit are mostly banded, fine- to medium-grained rocks while in the Upper Unit varieties with augen structures predominate. Orthogneisses show a dynamically recrystallised mineral assemblage of Qz + Pl + Bt ± Grt with accessory zircon and apatite. They are peraluminous (ASI = 1.20-1.27) and interpreted to belong to a high-K calc-alkaline suite of a VAG-type tectonic setting. LA-MC-ICP-MS U-Pb zircon data from samples from both units, from crystals with oscillatory zoning and Th/U > 0.1, yield similar concordia ages of ca. 534 Ma. This is interpreted to reflect the magmatic crystallization age of igneous precursors. These oldest meta-magmatics so far dated in the Western Tatra Mountains could be linked to the fragmentation of the northern margin of Gondwana. In zircons from a gneiss from the Upper Unit, cores with well-developed oscillatory zoning are surrounded by weakly luminescent, low contrast rims (Th/U < 0.1). These yield a concordia age of ca. 387 Ma corresponding to a subsequent, Eo-Variscan, high-grade metamorphic event, connected with the formation of crustal-scale nappe structures and collision-related magmatism.

CHRONOLOGICAL CONSTRAINTS ON FLUID CIRCULATION IN MESOZOIC FORMATIONS OF THE EASTERN PART OF THE PARIS BASIN INFERRED FROM U-Pb DATING OF SECONDARY INFILLING CARBONATES

NASA Astrophysics Data System (ADS)

Pisapia, C.; Deschamps, P.; Hamelin, B.; Buschaert, S.

2009-12-01

The French agency for nuclear waste management (ANDRA) developed an Underground Research Laboratory in the Mesozoic formations of Eastern part of the Paris Basin (France) to assess the feasibility of a high-level radioactive wastes repository in sedimentary formations. The target host formation is a low-porosity detrital argillite (Callovo-Oxfordian) embedded between two shelf limestones formations (of Bajocian-Bathonian and Oxfordian-Kimmeridgian ages). These formations are affected by fracture networks, likely inherited mainly from the Eocene-Oligocene extension tectonics, also responsible of the Rhine graben formation in the same region. The limestones have very low permeability, the primary and secondary porosity being infilled by secondary carbonated minerals. The inter-particle porosity is filled with euhedral calcite spar cements. Similarly, macro-cavities and connected micro-fractures are almost sealed by euhedral calcite. Geochemical evidences (δ18O) suggest that the secondary carbonates likely derived from a common parent fluid (Buschaert et al., 2004, Appl. Geochem. (19) 1201-1215p). This late carbonated precipitation phase is responsible for the intense cementation of the limestone formations and bears witness of a major phase of fluids circulation that marked the late diagenetic evolution of the system. Knowledge of the chronology of the different precipitation phases of secondary minerals is thus of critical importance in order to determine the past hydrological conditions of the geological site. The aim of this study is to provide chronological constraints on the secondary carbonate mineral precipitation using U/Th and U/Pb methods. Analyses are performed on millimeter to centimeter scale secondary calcites collected within fractures outcropping in the regional fault zone of Gondrecourt and in cores from the ANDRA exploration-drilling program. Preliminary U-Th analyses obtained on secondary carbonates from surface fractures infillings yield secular equilibrium composition, indicating that the precipitation phase was older than 650 ky. U-Pb measurements were performed on a VG sector Thermal Ionization Mass Spectrometer (TIMS) using a 205Pb-236U-233U-229Th spike. Pb contents are generally very low, between 3 and 20 ppb, while U contents are more variable, leading to μ = 238U/204Pb up to ~600. Sub-samples with high μ show radiogenic 206Pb/204Pb ratio, but at this stage isochrons generally show high scatter. These U-Pb data however are consistent with an Eocene-Oligocene period for the late carbonates precipitation phase. We will discuss the different processes that may be responsible for these errorchrons (i.e. heterogeneities in the initial isotopic composition; multi-stage growth) as well as the chronological constraints that can be drawn from these data.

A 2000-yr High-resolution Stalagmite Record From Zhenzhu Cave in Hebei, North China: Interpretations of AMS 14C, 230Th/U, 210Pb Dating, and δ18O, δ13C Results

NASA Astrophysics Data System (ADS)

Li, H. C.; Yin, J.; Rao, Z.; Mii, H. S.; Shen, C. C.; Pillutla, R. K.; Li, Y. X.

2016-12-01

An 11.1-cm long stalagmite (ZZ12) collected from Zhenzhu cave (38°15'N, 113°42'E, 975m a.s.l.) located at Tiangui mountain of Hebei province, North China. The 230Th/U dates on 12 horizons exhibit large uncertainties with many reversed age sequences due to low U contents and low 230Th/232Th ratios. While the 230Th/U dating is not able to provide the chronology of this stalagmite, AMS 14C dating on 27 samples from various depths of the stalagmite yields a reliable age-depth relationship. Three AMS 14C dates from the top 5 mm appear nuclear bomb carbon indicating that this part was deposited after AD 1950. Seven samples for 210Pb dating were taken from the upper 14 mm with 2 mm intervals, showing exponential decay of excess 210Pb and supporting the AMS 14C dating results. At the base of the stalagmite, charcoal grains were included in the carbonate stalagmite. This charcoal sample has a Calibrated 14C age of 1865±20 a BP. The carbonates at adjacent depths show Calibrated 14C ages of 1900±15 and 2215±75 a BP respectively. The bomb carbon and similar ages between the charcoal and carbonates indicate that dead carbon influence on the 14C dates in some horizons may not be serious. From the 27 AMS 14C dates, we select 17 AMS 14C dates which have minimal influence of dead carbon fraction to construct the chronology. The established chronology shows that slow growth rates occurred prior to 1100 a BP and after 600 a BP. This time interval involves the Medieval Warm Period, while the fast growth rate during this interval may reflect warm and wet climatic conditions. A total of 835 samples were drilled from the stalagmite for δ18O and δ13C analyses. The current 900-year δ18O and δ13C records reveal climate and vegetation changes in the study area. Strong decadal oscillations in the δ18O record reflect variations of monsoonal rain, with relatively dry between AD 1350 and AD 1550 and after AD 1960. The δ13C record appears mainly multi-centennial variations with a 4‰ fluctuation range and an average value of -8‰ (VPDB). The δ13C values were lighter than the average during AD 1150 1480 and AD 1700 1920, whereas the δ13C values were heavier than the average during AD 1480 1700 and AD 1820 the present. The δ13C record mainly reflects the vegetation change above the cave, with lighter value indicating better vegetation coverage and vice versa.

Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West-East Qiangtang collision.

PubMed

Zhang, Yu-Xiu; Jin, Xin; Zhang, Kai-Jun; Sun, Wei-Dong; Liu, Jian-Ming; Zhou, Xiao-Yao; Yan, Li-Long

2018-01-17

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure-temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U-Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West-East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

U-Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA

USGS Publications Warehouse

Aleinikoff, J.N.; Burton, W.C.; Lyttle, P.T.; Nelson, A.E.; Southworth, C.S.

2000-01-01

Mesoproterozoic granitic gneisses comprise most of the basement of the northern Blue Ridge geologic province in Virginia and Maryland. Lithology, structure, and U-Pb geochronology have been used to subdivide the gneisses into three groups. The oldest rocks, Group 1, are layered granitic gneiss (1153 ?? 6 Ma), hornblende monzonite gneiss (1149 ?? 19 Ma), porphyroblastic granite gneiss (1144 ?? 2 Ma), coarse-grained metagranite (about 1140 Ma), and charnockite (>1145 Ma?). These gneisses contain three Proterozoic deformational fabrics. Because of complex U-Pb systematics due to extensive overgrowths on magmatic cores, zircons from hornblende monzonite gneiss were dated using the sensitive high-resolution ion microprobe (SHRIMP), whereas all other ages are based on conventional U-Pb geochronology. Group 2 rocks are leucocratic and biotic varieties of Marshall Metagranite, dated at 1112??3 Ma and 1111 ?? 2 Ma respectively. Group 3 rocks are subdivided into two age groups: (1) garnetiferous metagranite (1077 ?? 4 Ma) and quartz-plagioclase gneiss (1077 ?? 4 Ma); (2) white leucocratic metagranite (1060 ?? 2 Ma), pink leucocratic metagranite (1059 ?? 2), biotite granite gneiss (1055 ?? 4 Ma), and megacrystic metagranite (1055 ?? 2 Ma). Groups 2 and 3 gneisses contain only the two younger Proterozoic deformational fabrics. Ages of monazite, seprated from seven samples, indicate growth during both igneous and metamorphic (thermal) events. However, ages obtained from individual grains may be mixtures of different age components, as suggested by backscatter electron (BSE) imaging of complexly zoned grains. Analyses of unzoned monazite (imaged by BSE and thought to contain only one age component) from porphyroblastic granite gneiss yield ages of 1070, 1060, and 1050 Ma. The range of ages of monazite (not reset to a uniform date) indicates that the Grenville granulite event at about 1035 Ma did not exceed about 750??C. Lack of evidence for 1110 Ma growth of monazite in porphyroblastic granite gneiss suggests that the Short Hill fault might be a Grenvillian structure that was reactivated in the Paleozoic. The timing of Proterozoic deformations is constrained by crystallization ages of the gneissic rocks. D1 occurred between about 1145 and 1075 Ma (or possibly between about 1145 and 1128 Ma). D2 and D3 must be younger than about 1050 Ma. Ages of Mesoproterozoic granitic rocks of the northern Blue Ridge are similar to rocks in other Grenville terranes of the eastern USA, including the Adirondacks and Hudson Highlands. However, comparisons with conventional U-Pb ages of granulite-grade rocks from the central and southern Appalachians may be specious because these ages may actually be mixtures of ages of cores and overgrowths.

In situ rutile petrochronology: texture-related T, Paleoproterozoic inheritance and a Pan-African overprint in the oldest subduction-related eclogites, Usagaran Orogen, Tanzania

NASA Astrophysics Data System (ADS)

Moeller, A.; Kraus, K.; Herms, P.; Appel, P.; Raase, P.

2014-12-01

Rutile U-Pb thermochronology is applied successfully by both TIMS and beam methods to date cooling events in mafic and metapelitic rocks, as well as in detrital studies. The Zr-in-rutile thermometer is very robust to thermal diffusion, and generally requires complete recrystallization to change recorded crystallization temperatures. Evidence for diffusion of HFSE elements in rutile is sparse; whereas U-Pb chronology generally records diffusion controlled cooling from the last event. This study follows conventional thermobarometry and U-Pb TIMS results on monazite, sphene and rutile of Möller et al. (1995) establishing a 2 Ga eclogite facies event from MORB-like metabasic, and metapelitic rocks in the Usagaran Orogen of Tanzania, interpreted to be the oldest outcrops of subduction-related eclogites. Rutile from both rock types were discordant near a ca. 500 Ma lower intercept, confirming a thermal overprint postulated on the basis of K-Ar and Rb-Sr mica ages by e.g. Wendt et al. (1972). The age of the eclogite-facies event was confirmed by U-Pb zircon dating of a 1991±2 Ma crosscutting pegmatite (Collins et al., 1999). We present in situ LA-ICP-MS rutile petrochronology on five metabasic and metapelitic eclogite facies samples with variable retrograde amphibolite-facies recrystallization. Thermometry confirms conventional Fe-Mg results, including higher peak temperatures in metabasites. Traverses on rutile inclusions in large garnet prophyroblasts in metapelites show increasing temperatures from cores outwards and a slight decrease towards outermost rims, with peak T coinciding with highest Mg# and highest grossular content, hence consistent with preservation of prograde zoning in the garnets and a brief eclogite facies event. Large rutiles (800μm) in recrystallized samples record temperature zoning profiles. U-Pb results show inheritance of near concordant 2 Ga domains, but dominantly confirm the ca. 490 Ma amphibolite facies overprint. The study is an excellent example of the potential of in-situ rutile petrochronology in complex, polymetamorphic rocks when meticulous attention is given to textural context. Möller et al., 1995, Geology, v. 23, p. 1067-1070. Collins et al., 2004, Earth Planet. Sci. Lett., v. 224, p. 175-192. Wendt et al., 1972, 24th Internat. Geol. Congr., Proc., p. 295-314.

Syn-collisional I-type Esenköy Pluton (Eastern Anatolia-Turkey): An indication for collision between Arabian and Eurasian plates

NASA Astrophysics Data System (ADS)

Açlan, Mustafa; Altun, Yusuf

2018-06-01

The Esenköy pluton which is situated in the East Anatolian Accretionary Complex (EACC) is represented by I-type, metalumino, calc-alkaline, VAG + syn-COLG, gabbro, diorite, quartz diorite, tonalite and granodiorite type rocks. This paper presents the characteristics of the above granitoids on their major, trace, rare earth elements (REE) and their zircon U-Pb dating. Zircon U-Pb crystallisation ages for gabbro, tonalite and granodiorite are 22.3 ± 0.2 Ma, 21.7 ± 0.2 Ma and 21.8 ± 0.2 Ma respectively. Esenköy granitoids show medium and high-K calc-alkaline character, with six exceptional K-poor sample plot in tholeiitic series field. The Rb/Y-Nb/Y diagram for Esenköy granitoids display subduction zone enrichment trend. The data which obtained from major, trace and REE geochemical characteristics and 206Pb/238U ages indicate that the collision which is take place between Arabian and Eurasian plates along the Bitlis-Zagros suture zone has begun in the Early Miocene (Aquitanian) or before from Early Miocene.

New geochronological history of the Mbuji-Mayi Supergroup (Proterozoic, DRC) through U-Pb and Sm-Nd dating

NASA Astrophysics Data System (ADS)

François, Camille; Baludikay, Blaise K.; Storme, Jean-Yves; Baudet, Daniel; Paquette, Jean-Louis; Fialin, Michel; Debaille, Vinciane; Javaux, Emmanuelle J.

2016-04-01

The Mbuji-Mayi Supergroup, DRC is located between the Archean-Paleoproterozoic Kasai Craton and the Mesoproterozoic Kibaran Belt. This sedimentary sequence, unaffected by regional metamorphism, preserves a large diversity of well-preserved acritarchs (organic-walled microfossils), evidencing the diversification of complex life (early eukaryotes) for the first time in mid-Proterozoic redox stratified oceans of Central Africa (Baludikay et al., in review). This Supergroup is composed of two distinct lithostratigraphic successions (i) BI Group: a lower siliciclastic sequence (ca. 1175 Myr to ca. 882 Myr or ca. 1050 Myr (Cahen, 1954; Holmes & Cahen, 1955; Delpomdor et al., 2013) unconformably overlying the ca. 2.82-2.56 Gyr granitoid Dibaya Complex to the North (Cahen & Snelling; recent notice on DRC geological map); and (ii) BII Group: a poorly age-constrained upper carbonate sequence with sparse shales . Basaltic lavas (including pillow lavas) overlying the Mbuji-Mayi Supergroup were dated around 950 Myr (Cahen et al., 1974; Cahen et al., 1984). To better constraint the age of this Supergroup in the Meso-Neoproterozoic limit, we combine different geochronological methods, in particular on diagenetic minerals such as monazite (Montel et al., 1996; Rasmussen & Muhling, 2007) and xenotime (McNaughton et al., 1999) but also on detrital zircons. For the BI Group, results of in situ U-Pb dating with LA-ICP-MS on monazite, xenotime and zircon (Laboratoire Magmas et Volcans, Clermont-Ferrand) provide ages between 2.9 and 1.2 Gyr for zircons and between 1.4 and 1.03 Gyr for monazites and xenotimes. New results of in situ U-Th-Pb dating of well-crystallized monazites and xenotimes with Electron MicroProbe (Camparis, UPMC, Paris), highlight that some crystals display zonations with an inherited core older than 1125 Myr and diagenetic rims around 1050-1075 Myr. This suggests that the diagenesis of BI Group is younger than 1175 Myr (Delpomdor et al., 2013) and probably around 1030-1075 Myr, coherent with an age on 2 syngenetic galenas around 1055 Myr for the top of BI Group (Cahen, 1954; Holmes & Cahen, 1955). Sm-Nd datings on basaltic pillow lavas overlying the Mbuji-Mayi Supergroup (previously dated around 950 Myr (Cahen et al., 1974; Cahen et al., 1984) are in progress (Laboratoire G-Time, ULB, Bruxelles) to precisely limit in time the end of deposition of this Supergroup.

Search for diphoton events with large missing transverse energy with 36 pb –1 of 7 TeV proton–proton collision data with the ATLAS detector

DOE PAGES

Aad, G.; Abbott, B.; Abdallah, J.; ...

2011-10-05

Making use of 36 pb –1 of proton–proton collision data at √s = 7 TeV, the ATLAS Collaboration has performed a search for diphoton events with large missing transverse energy. Observing no excess of events above the Standard Model prediction, a 95% Confidence Level (CL) upper limit is set on the cross section for new physics of σ<0.38–0.65 pb in the context of a generalised model of gauge-mediated supersymmetry breaking (GGM) with a bino-like lightest neutralino, and of σ<0.18–0.23 pb in the context of a specific model with one universal extra dimension (UED). A 95% CL lower limit of 560more » GeV, for bino masses above 50 GeV, is set on the GGM gluino mass, while a lower limit of 1/ R>961 GeV is set on the UED compactification radius R. In conclusion, these limits provide the most stringent tests of these models to date.« less

Zircon U-Pb and molybdenite Re-Os geochronology and Sr-Nd-Pb-Hf isotopic constraints on the genesis of the Xuejiping porphyry copper deposit in Zhongdian, Northwest Yunnan, China

NASA Astrophysics Data System (ADS)

Leng, Cheng-Biao; Zhang, Xing-Chun; Hu, Rui-Zhong; Wang, Shou-Xu; Zhong, Hong; Wang, Wai-Quan; Bi, Xian-Wu

2012-10-01

The Xuejiping porphyry copper deposit is located in northwestern Yunnan Province, China. Tectonically, it lies in the southern part of the Triassic Yidun island arc. The copper mineralization is mainly hosted in quartz-dioritic and quartz-monzonitic porphyries which intruded into clastic-volcanic rocks of the Late Triassic Tumugou Formation. There are several alteration zones including potassic, strong silicific and phyllic, argillic, and propylitic alteration zones from inner to outer of the mineralized porphyry bodies. The ages of ore-bearing quartz-monzonitic porphyry and its host andesite are obtained by using the zircon SIMS U-Pb dating method, with results of 218.3 ± 1.6 Ma (MSWD = 0.31, N = 15) and 218.5 ± 1.6 Ma (MSWD = 0.91, N = 16), respectively. Meanwhile, the molybdenite Re-Os dating yields a Re-Os isochronal age of 221.4 ± 2.3 Ma (MSWD = 0.54, N = 5) and a weighted mean age of 219.9 ± 0.7 Ma (MSWD = 0.88). They are quite in accordance with the zircon U-Pb ages within errors. Furthermore, all of them are contemporary with the timing of the Garzê-Litang oceanic crust subduction in the Yidun arc. Therefore, the Xuejiping deposit could be formed in a continental margin setting. There are negative ɛNd(t) values ranging from -3.8 to -2.1 and relatively high initial 87Sr/86Sr ratios from 0.7051 to 0.7059 for the Xuejiping porphyries and host andesites. The (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t values of the Xuejiping porphyries and host andesites vary from 17.899 to 18.654, from 15.529 to 15.626, and from 37.864 to 38.52, respectively, indicative of high radiogenic Pb isotopic features. In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS exhibit that there are quite uniform and slightly positive ɛHf(t) values ranging from -0.2 to +3.2 (mostly between 0 and +2), corresponding to relatively young single-stage Hf model ages from 735 Ma to 871 Ma. These isotopic features suggest that the primary magmas of the Xuejiping porphyries and their host andesites were mainly derived from a metasomatized mantle, with contamination of about 5-10% crustal rocks during ascending. Comparing with typical porphyry Cu deposits, the Xuejiping porphyry Cu deposit is distinct by strong silicific and phyllic alteration and major stockwork veining mineralization in the ore-bearing porphyries, but lack of pervasive potassic alteration and disseminated mineralization. This indicates that there could be more prospective Cu resources in the Xuejiping ore district.

CL-imaging and ion microprobe dating of single zircons from a high-grade rock from the Central Zone, Limpopo Belt, South Africa: Evidence for a single metamorphic event at ˜2.0 Ga

NASA Astrophysics Data System (ADS)

Mouri, H.; Brandl, G.; Whitehouse, M.; de Waal, S.; Guiraud, M.

2008-02-01

The combination of ion microprobe dating and cathodoluminescence (CL) imaging of zircons from a high-grade rock from the Central Zone of the Limpopo Belt were used to constrain the age of metamorphic events in the area. Zircon grains extracted from an orthopyroxene-gedrite-bearing granulite were prepared for single crystal CL-imaging and ion microprobe dating. The grains display complex zoning when using SEM-based CL-imaging. A common feature in most grains is the presence of a distinct core with a broken oscillatory zoned structure, which clearly appears to be the remnant of an original grain of igneous origin. This core is overgrown by an unzoned thin rim measuring about 10-30 μm in diameter, which is considered as new zircon growth during a single metamorphic event. Selected domains of the zircon grains were analysed for U, Pb and Th isotopic composition using a CAMECA IMS 1270 ion microprobe (Nordsim facility). Most of the grains define a near-concordant cluster with some evidence of Pb loss. The most concordant ages of the cores yielded a weighted mean 207Pb/ 206Pb age of 2689 ± 15 (2 σ) Ma, interpreted as the age of the protolith of an igneous origin. The unzoned overgrowths of the zircon grains yielded a considerably younger weighted mean 207Pb/ 206Pb age of ˜2006.5 ± 8.0 Ma (2 σ), and these data are interpreted to reflect closely the age of the ubiquitous high-grade metamorphic event in the Central Zone. This study shows clearly, based on both the internal structure of the zircons and the data obtained by ion microprobe dating, that only a single metamorphic event is recorded by the studied 2.69 Ga old rocks, and we found no evidence of an earlier metamorphic event at ˜2.5 Ga as postulated earlier by some workers.

Age of metamorphic events : petrochronology and hygrochronology

NASA Astrophysics Data System (ADS)

Bosse, Valerie; Villa, Igor M.

2017-04-01

Geodynamic models of the lithosphere require quantitative data from natural samples. Time is a key parameter: it allows to calculate rates and duration of geological processes and provides informations about the involved physical processes (Vance et al. 2003). Large-scale orogenic models require linking geochronological data with other parameters: structures, kinematics, magmatic and metamorphic petrology (P-T-A-X conditions), thermobarometric evolution of the lithosphere, chemical dynamics (Muller, 2003). This requires geochronometers that are both powerful chemical and petrological tracers. In-situ techniques allow dating a mineral in its petrological-microstructural environment. Getting a "date" has become quite easy... But what do we date in the end ? What is the link between the numbers obtained from the mass spectrometer and the age of the metamorphic event we are trying to date ? How can we transform the date into a geological meaningful age ? What do we learn about the behavior of the geochronometer minerals? Now that we can perform precise dating on very small samples directly in the studied rock, it is important to improve the way we interpret the ages to give them more pertinence in the geodynamic context. We propose to discuss the Th/U/Pb system isotopic closure in various metamorphic contexts using our published examples of in situ dating on monazite and zircon (Bosse et al. 2009; Didier et al. 2014, 2015). The studied examples show that (i) fluid assisted dissolution-precipitation processes rather than temperature-dependent solid diffusion predominantly govern the closure of the Th/U/Pb system (ii) monazite and zircon are sensitive to the interaction with fluids of specific composition (F, CO2, K ...), even at low temperature (iii) in the absence of fluids, monazite is able to record HT events and to retain this information in poly-orogenic contexts or during partial melting events (iv) complex chemical and isotopic zonations, well known in monazite, reflect the interaction with the surrounding mineral assemblages. An often neglected observation is that the K-Ar chronometer minerals show similar patterns of isotopic inheritance closely tied to relict patches and heterochemical retrogression phases (Villa and Williams 2013). Isotopic closure in the U-Pb and K-Ar systems follows the same principle: thermal diffusion is very slow, dissolution and reprecipitation are several orders of magnitude faster. This means that both U-Pb and K-Ar mineral chronometers are hygrochronometers. The interpretation of the ages of the different domains cannot be decoupled from the geochemical and petrological context. The focus on petrology also requires, following Villa (1998, 2016), that the ages measured in metamorphic rocks no longer can be used in geodynamic models according to the "closure temperature" concept as originally defined by Dodson (1973). Bosse et al. (2009) Chem Geol 261: 286 Didier et al. (2014) Chem Geol 381: 206 Didier et al. (2015) Contrib Mineral Petrol 170: 45 Dodson (1973) Contrib Mineral Petrol 40: 259 Muller (2003) EPSL, 206: 237 Villa (1998) Terra Nova 10: 42 Villa (2016) Chem Geol 420: 1 Villa & Williams (2013) In: Harlov & Austrheim (eds.), Metasomatism and the Chemical Transformation of Rock. Springer, p171

Alteration and chemical U-Th-total Pb dating of heterogeneous high-uranium zircon from a pegmatite from the Aduiskii massif, middle Urals, Russia

NASA Astrophysics Data System (ADS)

Zamyatin, Dmitry A.; Shchapova, Yuliya V.; Votyakov, Sergey L.; Nasdala, Lutz; Lenz, Christoph

2017-09-01

The U-Th-Pb isotope system in the accessory mineral zircon may be disturbed, as for instance by the secondary loss of radiogenic lead. The recognition of such alteration is crucial for the sound interpretation of geochronology results, in particular for chemical dating by means of an electron probe micro-analyser (EPMA). Here we present the example of high-U zircon samples from a granite pegmatite from the Aduiskii Massif, Middle Urals, Russia. The structural and chemical heterogeneity of samples was characterised by EPMA, including joint probability distribution (JPD) analysis of back-scattered electrons (BSE), cathodoluminescence (CL) and U M β images, and by Raman and photoluminescence (PL) spectroscopy. We found a high-U interior region (U up to 11.4 wt%) without any obvious indication of alteration. This domain has stoichiometric composition, and its Raman spectrum is similar to that of amorphous ZrSiO4. In addition, altered lower-U regions are present that are non-stoichiometric and contain non-formula elements such as Ca, Al, Fe, and water up to several wt%. Their Raman spectra yielded a band near 760-810 cm-1 which is not related to any ZrSiO4 vibration; we assign it tentatively to the symmetric stretching of (UO2)2+ groups. This assignment is supported by the observation of a fairly intense PL phenomenon whose spectral position and vibrational-coupling structure strongly indicates a uranyl-related emission. Altered zones were formed by both fluid-driven diffusion reaction and coupled dissolution-reprecipitation processes. The variation of BSE and CL intensities in amorphous high-U zircon is controlled by its chemical composition and the presence of water and uranyl groups. We have determined a weighted mean EPMA age of 246 ± 2 Ma, which agrees reasonably well with previous dating results for the Aduiskii Massif.

Ar-39 - Ar-40 Dating of Two Angrites and Two Brachnites

NASA Technical Reports Server (NTRS)

Garrison, Daniel; Bogard, Donald

2003-01-01

Angrites are a rare group (approx.7 known) of igneous meteorites with basalt-like composition, which probably derive from a relatively small parent body that differs from those of other igneous meteorites. Angrites show evidence for extinct Mn-53, Sm-146, and Pu-244, and precise U-Pb, and Pb-Pb ages of 4.558 Gyr for two angrites define the time of early parent body differentiation. The S-147 - Nd-143 ages of two angrites range between 4.53+/-0.04 and 4.56+/-0.04 Gyr, but no Ar-39 - Ar-40 or Rb-Sr ages have been reported. Most angrites show no evidence for either shock brecciation or metamorphism.

Computer Programs in Marine Science

DTIC Science & Technology

1976-04-01

AD-A279 795 U.S. DEPARTMENT OF COMMERCE National Technical Information Service PB-258 082 Computer Programs in Marine Science National Ocearncgraphic...NO. 5 fo r- Computer nPrograms in ’StlrSO " Marine Science U.S. DEPARTMENT OF COMMERCE National Oceanic and AtmosPheric AdmInistration Environmental...N0,AA?76062212 I I 4. TITLE A?.’D.UBTITLE S. REPORT DATE Comnuter Progrims in Marine Science April 1976 Koy tc Oceanographic Records Documentation No

Assessing the 40K decay constant by intercalibration with U-Pb, Rb-Sr and K-Ca chronometers

NASA Astrophysics Data System (ADS)

Naumenko-Dèzes, M. O.; Nagler, T. F.; Mezger, K.; Villa, I. M.

2016-12-01

Ar-Ar is one of the most used dating systems and its accuracy plays an important role in constraining the age of planets, durations of processes and their sequence of occurrence. This system has been reported to give ages that are ca. 1% younger than U-Pb ages. The discrepancies between the two mostly used and precise geochronometers, U-Pb and K-Ar, have been a subject of critical reviewing (e.g. Renne et al. [1] and ref. therein) and were attributed to a systematic offset of the 40K decay constant. Multiple attempts to recalibrate it did not achieve consistency.We attempted to intercalibrate three dating systems: U-Pb (the main reference), Rb-Sr (the consistency check) and K-Ca (the unknown). We examined 11 natural samples, but only a phlogopite from the Phalaborwa carbonatite complex met all requirements of a geological "point-like" event [2]. The Rb-Sr age of this sample is 2058.9±5.2 Ma and agrees with the age determined by Nebel et al [3] and with published U-Pb ages. The K-Ca age for the same sample calculated with constants of Steiger&Jäger [4] is 2040±13 Ma, ca.1% younger age than U-Pb reference age of the complex. Since we improved the Ca measurements [5] the K-Ca age has a low uncertainty of 0.6%. This age constrains the decay constant of the Ca branch of the 40K decay. However, the total decay constant is given by one equation with two unknowns, the branching ratio B and the decay constant of the Ar branch.Within the range of published branching ratios (B = 0.892 to 0.896 [6]) the best total 40K decay constant lies mid-way between the values proposed by Renne et al [7] and Min et al [8] (fig. 1).Figure 1. K-Ca age of Phalaborwa phlogopite changes along sloping lines as a function of assumed branching ratio B, calculated with the total 40K decay constant of [3]. [5], [6].[1] Renne et al. (2010) Geochim. Cosmochim Acta 74, 5349-5367; [2] Begemann et al. (2001) Geochim. Cosmochim Acta 65, 111-121; [3] Nebel et al (2010) GCA 74, 5349; [4] Steiger&Jäger (1977) Earth and Plan. Sci. L. 36, 359-362; [5] Naumenko-Dèzes et al. (2015) Int. J. of Mass Spectr. 387, 60-68; [9] Bé et al (2004) Table of Radionuclides, A=1 to 150; [7] Renne et al. (2011) Geochim. et Cosmochim. Acta 75, 5097-5100; [8] Min et al. (2000) Geochim. et Cosmochim. Acta 64, 73-9.

A Complication in Determining the Precise Age of the Solar System

NASA Astrophysics Data System (ADS)

Brennecka, G. A.

2010-01-01

Primitive components in meteorites contain a detailed record of the conditions and processes in the solar nebula, the cloud of dust and gas surrounding the infant Sun. Determining accurately when the first materials formed requires the lead-lead (Pb-Pb) dating method, a method based on the decay of uranium (U) isotopes to Pb isotopes. The initial ratio of U-238 to U-235 is critical to determining the ages correctly, and many studies have concluded that the ratio is constant for any given age. However, my colleagues at Arizona State University, Institut fur Geowissenschaften, Goethe-Universitat (Frankfurt, Germany), and the Senckenberg Forschungsinstitut und Naturmuseum (also in Frankfurt) and I have found that some calcium-aluminum-rich inclusions (CAIs) in chondritic meteorites deviate from the conventional value for the U-238/U-235 ratio. This could lead to inaccuracies of up to 5 million years in the age of these objects, if no correction is made. Variations in the concentrations of thorium and neodymium with the U-238/U-235 ratio suggest that the ratio may have been lowered by the decay of curium-247, which decays to U-235 with a half-life of 15.6 million years. Curium-247 is created in certain types of energetic supernovae, so its presence suggests that a supernova added material to the pre-solar interstellar cloud between 110 and 140 million years before the Solar System began to form.

National geochronological and natural radioelement data bases

USGS Publications Warehouse

Zartman, Robert E.; Bush, Charles A.; Abston, C.C.

1995-01-01

This CD-ROM contains both the National Geochronological Data Base [NGDB] and the Natural Radioelement Data Base [NRDB]. Supporting location, geologic, and reference information is provided for both data bases. The NGDB is a compilation of more than 30,000 individual published Pb-alpha, fission-track, K-Ar, Rb-Sr, U-Th-Pb, and Sm-Nd rock and mineral ages reported on approximately 18,000 dated samples from the United States. A program is provided to search the data files by latitude and longitude, state, analytical method, and age range. The NGDB is provided as quote-comma delimited files that can be entered into most commercial spreadsheet programs. The NRDB gives gamma-ray spectrometric analyses of the natural radioelements (U, Th, and K) for more than 8500 whole-rock samples obtained under the USGS Natural Radioelement Distribution Project. A program is provided to search the data files by state, keyword, U content, Th content, and K content.

Detrital zircon ages from southern Norway - implications for the Proterozoic evolution of the southwestern Baltic Shield

NASA Astrophysics Data System (ADS)

Knudsen, T.-L.; Andersen, T.; Whitehouse, M. J.; Vestin, J.

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1Ga, although there is no record of rocks older than 1.6Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have 207Pb/206Pb ages between 1367 and 1939Ma, with frequency maxima in the range 1.85 to 1.70Ga and 1.60 to 1.50Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core 207Pb/206Pb age of 2383 Ma. Two discrete, unzoned zircons have 207Pb/206Pb ages of 1122 and 1133Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent ``inner cores'' which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant 206Pb/238U versus 207Pb/235U ratios and maximum 207Pb/206Pb ages of 1438Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield.

A novel approach to in-situ rutile petrochronology

NASA Astrophysics Data System (ADS)

Kooijman, Ellen; Smit, Matthijs; Kylander-Clark, Andrew

2017-04-01

Rutile petrochronology has become an increasingly important tool for deciphering the timing and conditions of petrological processes. Rutile provides a reliable single-mineral thermometer, capable of retaining temperature information during high and ultra-high temperature metamorphism. Its HFSE contents can be used to investigate the geochemical environment in which rutile crystallized. Most importantly, rutile strongly fractionates U/Pb and enables U-Pb thermochronology in the intermediate temperature range. Here we present a novel approach to using U-Pb thermochronology of rutile by exploring the use of Pb as a diffusive species in kinetics-based thermometry. We performed high spatial and analytical resolution micro-analysis of rutile by laser ablation multi-collector ICPMS to constrain Pb diffusion profiles in rutile from high-grade metamorphic rocks of the Western Gneiss Complex (WGC), Norway. The age and thermometric results from this analysis are used to constrain a full thermal history from single grains. Millimeter-sized single crystals of rutile from a rutile-rich phlogopitite vein in eclogite were mounted and polished to expose their geometric cores. The grains were analyzed in transects using rectangular spots (c. 15x45 μm). This ensures ablation of a significant volume while maintaining the required radial spatial resolution. The transects yielded well-defined Pb diffusion profiles, with U-Pb ages ranging from c. 415 Ma in the cores to c. 380 Ma in the outermost rims (±2%, 2σ on individual spots). Diffusion zoning length was used with well-established Pb diffusion parameters [1] to determine peak temperature conditions following the approach of [2]. The result, c. 810 ± 25 ˚ C, is consistent with 800 ± 25 ˚ C and c. 780 ˚ C estimated for the same sample using conventional and Zr-in-rutile thermometry, respectively. The cooling history that is reconstructed through age zoning analysis and diffusion modeling shows remarkable consistency with that established for the WGC through decades of 40Ar/39Ar dating. The data presented here demonstrate that in-situ rutile U-Pb analysis yields reliable and precise temperature and age information that can be combined to resolve full thermal histories from single crystals. This novel approach to the toolbox of rutile petrochronology has great potential for research into the tectonics and dynamics of the lithosphere. References [1] Cherniak (2000) Contrib. Mineral. Petrol. 139. 198-207. [2] Smit et al. (2013) J. Metamorph. Geol. 31. 339-358.

Influence of accretion on lead in the Earth

NASA Astrophysics Data System (ADS)

Galer, Stephen J. G.; Goldstein, Steven L.

The Pb abundance and isotope composition of the Earth is fundamentally altered from bulk solar system values by the processes occurring during accretion. The most important of the possible processes are volatile element loss and core formation, or some form of inhomogeneous accretion/condensation. The final result is an Earth highly impoverished in 204Pb and other Pb isotopes in primordial abundance. Depending on the exact timing, some radiogenic Pb is also lost either to space or to the core; the degree of loss occurs in the same order as the parent decay constants, namely 207Pb > 206Pb > 208Pb. In this contribution, we explore the likely effects accretion had on the Pb isotope composition of the present day bulk silicate Earth and its secular isotope evolution. This is used to address a number of questions: (1) What can be learned about accretion from the Pb isotope composition of the bulk silicate Earth? (2) Can effects of accretion reconcile the classical "Pb paradox" of a 206Pb-rich bulk silicate Earth? (3) What exactly is the meaning of the "age of the Earth" within the context of Pb isotopes? By consideration of a number of accretion scenarios it is demonstrated that Pb isotopes yield information only on the following two coupled quantities: Firstly, the accretion interval Δ T, the time between initial condensation of the solar nebula (at 4.566Ga) and when accretion-produced U/Pb fractionation (whether loss of Pb to the core or to space) in the silicate Earth ceased. Secondly, the mean 238U/204 Pb ratio μ during accretion—no details of changes in μ during the accretion interval can be resolved. The effects of accretion are thus adequately considered in terms of a simple two-stage model described by μ over ΔT followed by a postaccretion μ. The systematics of μ and ΔT are then examined for the cases of present day terrestrial reservoirs and Archean leads. These estimates of μ and ΔT for the present and past silicate Earth are not compatible with ΔT = 0; rather, they require ΔT ≥ 50Ma and μ ≥ 0 in all instances, with our best estimate of ΔT being 80±40Ma. From a number of lines of argument it can be demonstrated that the U-Pb "age of the Earth" records an endogenous process actively taking place during accretion. Further, this process cannot be volatile loss of Pb, but rather it actually records the termination of Pb partitioning into the core. This does not necessarily date the endpoint of growth of the Earth for two reasons: Firstly, this core formation "age" may itself in part reflect that occurring on bodies later contributing to the Earth; secondly, later infall of bodies may alter μ but leave the U-Pb "age" of the silicate Earth effectively unaltered. Overall, the Earth can be considerably `younger' than previous single-stage model U-Pb ages for the silicate Earth have suggested. In addition, the "lead paradox" is seen to be a natural consequence of the finite time taken for accretion and core formation on the Earth.

Paleomagnetism and geochronological studies on a 450 km long 2216 Ma dyke from the Dharwar craton, southern India

NASA Astrophysics Data System (ADS)

Nagaraju, E.; Parashuramulu, V.; Kumar, Anil; Srinivas Sarma, D.

2018-01-01

Paleomagnetic and geochronological studies were carried out on a ∼ 450 km long (from 17 sites) N-S striking Paleoproterozoic dyke swarm exposed along a natural crustal cross section of about 10 km (increasing from North to South) in the Dharwar Craton, to study the characteristics of paleomagnetism and geochronology in vertical dimension. U-Pb/Pb-Pb dating on baddeleyite gives a crystallisation age of 2216.0 ± 0.9 Ma for long dyke AKLD. Paleomagnetic data from this well dated ∼ 2216 Ma dyke swarm in Dharwar Craton are of excellent quality. High coercivity and high blocking temperature components are carried by single domain magnetite and show dual polarity remanence directions. Combined normal and reverse polarity remanences on AKLD and other N-S dykes define the most reliable paleomagnetic pole for ∼ 2216 Ma at latitude 36°S and longitude 312°E (A95 = 7°). Though paleomagnetic data is unavailable on other N-S dykes below the Cuddapah basin, high precision geochronology suggest that they are of similar age within errors. Though there is a variation in the crustal depth of Dharwar craton from north to south, consistent Pb-Pb/U-Pb baddeleyite geochronology and paleomagnetic studies along the AKLD established its continuity and preservation along its entire strike length. The virtual geomagnetic poles of these sites confirm a stable remanence and are almost identical to the previously reported paleomagnetic pole and also supported by positive reversal test. Positive paleomagnetic reversal test on these dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. Updated apparent polar wander path of Dharwar craton indicates relatively low drift rate during 2.21-2.08 Ga interval. Magnetogranulometry and SEM studies show that remanent magnetization in this dyke was carried by single domain magnetite residing within silicate minerals.

Using T2-Exchange from Ln3+DOTA-Based Chelates for Contrast-Enhanced Molecular Imaging of Prostate Cancer with MRI

DTIC Science & Technology

2016-04-01

group meetings to help with collaboration among the group members. He also gave two conference presentations (see Section 6) about this research ...Dallas Dallas, TX 75390-8568 REPORT DATE: April 2016 TYPE OF REPORT: Final PREPARED FOR: U.S. Army Medical Research and Materiel Command Fort Detrick...SPONSOR/MONITOR’S ACRONYM(S) U.S. Army Medical Research and Materiel Command Fort Detrick, Maryland 21702-5012 11. SPONSOR/MONITOR’S REPORT NUMBER

Empirical constraints on the effects of radiation damage on helium diffusion in zircon

NASA Astrophysics Data System (ADS)

Anderson, Alyssa J.; Hodges, Kip V.; van Soest, Matthijs C.

2017-12-01

In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523 Ma and 88.0 to 138.9 Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474 Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520 Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15 × 1019 α /g, spanning the complete damage spectrum. We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple mineral-isotopic systems should be employed to add additional, independent constraints to a sample's thermal history.

Anthropogenic contributions to atmospheric Hg, Pb and As accumulation recorded by peat cores from southern Greenland and Denmark dated using the 14C "bomb pulse curve"

NASA Astrophysics Data System (ADS)

Shotyk, W.; Goodsite, M. E.; Roos-Barraclough, F.; Frei, R.; Heinemeier, J.; Asmund, G.; Lohse, C.; Hansen, T. S.

2003-11-01

Mercury concentrations are clearly elevated in the surface and sub-surface layers of peat cores collected from a minerotrophic ("groundwater-fed") fen in southern Greenland (GL) and an ombrotrophic ("rainwater-fed") bog in Denmark (DK). Using 14C to precisely date samples since ca. AD 1950 using the "atmospheric bomb pulse," the chronology of Hg accumulation in GL is remarkably similar to the bog in DK where Hg was supplied only by atmospheric deposition: this suggests not only that Hg has been supplied to the surface layers of the minerotrophic core (GL) primarily by atmospheric inputs, but also that the peat cores have preserved a consistent record of the changing rates of atmospheric Hg accumulation. The lowest Hg fluxes in the GL core (0.3 to 0.5 μg/m 2/yr) were found in peats dating from AD 550 to AD 975, compared to the maximum of 164 μg/m 2/yr in AD 1953. Atmospheric Hg accumulation rates have since declined, with the value for 1995 (14 μg/m 2/yr) comparable to the value for 1995 obtained by published studies of atmospheric transport modelling (12 μg/m 2/yr). The greatest rates of atmospheric Hg accumulation in the DK core are also found in the sample dating from AD 1953 and are comparable in magnitude (184 μg/m 2/yr) to the GL core; again, the fluxes have since gone into strong decline. The accumulation rates recorded by the peat core for AD 1994 (14 μg/m 2/yr) are also comparable to the value for 1995 obtained by atmospheric transport modelling (18 μg/m 2/yr). Comparing the Pb/Ti and As/Ti ratios of the DK samples with the corresponding crustal ratios (or "natural background values" for preanthropogenic peat) shows that the samples dating from 1953 also contain the maximum concentration of "excess" Pb and As. The synchroneity of the enrichments of all three elements (Hg, Pb, and As) suggests a common source, with coal-burning the most likely candidate. Independent support for this interpretation was obtained from the Pb isotope data ( 206Pb/ 207Pb = 1.1481 ± 0.0002 in the leached fraction and 1.1505 ± 0.0002 in the residual fraction) which is too radiogenic to be explained in terms of gasoline lead alone, but compares well with values for U.K. coals. In contrast, the lowest values for 206Pb/ 207Pb in the DK profile (1.1370 ± 0.0003 in the leached fraction and 1.1408 ± 0.0003 in the residual fraction) are found in the sample dating from AD 1979: this shows that the maximum contribution of leaded gasoline occurred approximately 25 yr after the zenith in total anthropogenic Pb deposition.

Evolution of Palaeoproterozoic mafic intrusions located within the thermal aureole of the Sudbury Igneous Complex, Canada: Isotopic, geochronological and geochemical evidence

NASA Astrophysics Data System (ADS)

Prevec, Stephen A.; Baadsgaard, Halfdan

2005-07-01

Impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth. Age-dating of impact events is critical in correlating cause and effects for these catastrophic processes. The Falconbridge and Drury Township (Twp) intrusions were emplaced at the contact between Neoarchaean basement and Palaeoproterozoic volcanosedimentary rocks, and also lie at the southeast and southwest edges of the Sudbury Igneous Complex (SIC), within its thermal contact aureole. The Falconbridge Twp intrusion is dated at 2441 ± 3 Ma by U-Pb zircon, with evidence of Archaean inheritance from its host granitoids. Granitoids from the southernmost Abitibi Subprovince are dated here between 2670 ± 11 Ma for an undeformed Algoman granite, and 2696 ± 18 Ma for a foliated granitoid, consistent with existing data from the Abitibi Greenstone Belt and from the Wawa Subprovince. Major and trace element geochemical evidence, common-Pb isotopic compositions, and ɛNd2440 values between 0 and -1 are all consistent with a Palaeoproterozoic origin for the Falconbridge Twp intrusion, and support inclusion in the East Bull Lake-type suite of leucogabbroic plutons and sills. In contrast, the Drury Twp intrusion gives a U-Pb zircon age of 1859 ± 13 Ma, coincident with the date of SIC-emplacement. While the major and trace element compositions are comparable to the Falconbridge data, the Drury displays significant heterogeneity in ɛNd2440, with values ranging from +3.7 to -0.1, and contains more radiogenic Pb isotopic compositions. Field, geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the SIC itself, and indicates that the Drury too is a Palaeoproterozoic intrusion. This requires that apparently unshocked, undeformed magmatic-looking zircon has been grown or reset in a postmagmatic setting. This has significant implications for the identification of mantle-derived magmas and crustal remelts associated with large impact craters. A resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen-complexing is proposed, inducing rapid, postshock lead loss and subsequent annealing.

Zircon U-Pb and molybdenite Re-Os geochronology and geological significance of the Baoshan porphyry Cu polymetallic deposit in Jiangxi province

NASA Astrophysics Data System (ADS)

Jia, Liqiong; Wang, Liang

2017-10-01

Baoshan porphyry Cu polymetallic deposit belongs to Jiujiang-Ruichang Cu-Au ore field, which is a component part of the Middle-Lower Yangtze River Cu-Au metallogenic belt. The U-Pb LA-MC-TCP MS dating of the zircons from Baoshan granodiorite porphyry yields an age of 147.81±0.48Ma (MSWD=1.07). Six molybdenite samples separated from Baoshan deposit are used for Re-Os dating and obtained the weighted average age of 147.42±0.84Ma and an isochron age of 147.7±1.2Ma. These ages suggest that the mineralization in the Baoshan deposit is genetically associated to the granodiorite porphyry, and the process of rock-and ore-forming is continuous. These data indicate that ages of intrusion and ore-body from Baoshan deposit are almost identical to other typical magmatic intrusion and deposits in Jiujiang-Ruichang metallogenic district. Tt is inferred that the Baoshan deposit was formed in the transition from EW-striking Tndosinian tectonic domain to NE-striking Paleo-Pacific tectonic domain.

Tectono-magmatic evolution of the Chihuahua-Sinaloa border region in northern Mexico: Insights from zircon-apatite U-Pb geochronology, zircon Hf isotope composition and geochemistry of granodiorite intrusions

NASA Astrophysics Data System (ADS)

Mahar, Munazzam Ali; Goodell, Philip C.; Feinstein, Michael Nicholas

2016-11-01

We present the whole-rock geochemistry, LA-ICP-MS zircon-apatite U-Pb ages and zircon Hf isotope composition of the granodioritic plutons at the southwestern boundary of Chihuahua with the states of Sinaloa and Sonora. These granodiorites are exposed in the north and south of the Rio El Fuerte in southwest Chihuahua and northern Sinaloa. The magmatism spans over a time period of 37 Ma from 90 to 53 Ma. Zircons are exclusively magmatic with strong oscillatory zoning. No inheritance of any age has been observed. Our new U-Pb dating ( 250 analyses) does not support the involvement of older basement lithologies in the generation of the granitic magmas. The U-Pb apatite ages from granodiorites in southwest Chihuahua vary from 52 to 70 Ma. These apatite ages are 1 to 20 Ma younger than the corresponding zircon U-Pb crystallization ages, suggesting variable cooling rates from very fast to 15 °C/Ma ( 800 °C to 500 °C) and shallow to moderate emplacement depths. In contrast, U-Pb apatite ages from the Sinaloa batholith are restricted from 64 to 61 Ma and are indistinguishable from the zircon U-Pb ages range from 67 to 60 Ma within the error, indicating rapid cooling and very shallow emplacement. However, one sample from El Realito showed a larger difference of 20 Ma in zircon-apatite age pair: zircon 80 ± 0.8 Ma and apatite 60.6 ± 4 Ma, suggesting a slower cooling rate of 15 °C/Ma. The weighted mean initial εHf (t) isotope composition (2σ) of granodiorites varies from + 1.8 to + 5.2. The radiogenic Hf isotope composition coupled with previous Sr-Nd isotope data demonstrates a significant shift from multiple crustal sources in the Sonoran batholithic belt to the predominant contribution of the mantle-derived magmas in the southwest Chihuahua and northern Sinaloa. Based on U-Pb ages, the absence of inheritance, typical high Th/U ratio and radiogenic Hf isotope composition, we suggest that the Late Cretaceous-Paleogene magmatic rocks in this region are not derived from melting of a felsic older crust, neither evolved North American Proterozoic basement nor Jurassic metasedimentary rocks of the Guerrero terrane. Instead, the magma was primarily derived from partial melting of mantle related sources, possibly in the subcontinental mantle wedge above the Farallon plate. Our younger U-Pb zircon-apatite ages are well correlated with the late-stage cogenetic porphyry Cu-Au mineralization at 57 to 59 Ma. If the origin of ore mineralization is related to the magma sources of host batholiths, then a minimal input from crustal melt is suggested in the genesis of Cu-Au porphyry system.

High-precision U-Pb geochronologic constraints on the Late Cretaceous terrestrial cyclostratigraphy and geomagnetic polarity from the Songliao Basin, Northeast China

NASA Astrophysics Data System (ADS)

Wang, Tiantian; Ramezani, Jahandar; Wang, Chengshan; Wu, Huaichun; He, Huaiyu; Bowring, Samuel A.

2016-07-01

The Cretaceous continental sedimentary records are essential to our understanding of how the terrestrial geologic and ecologic systems responded to past climate fluctuations under greenhouse conditions and our ability to forecast climate change in the future. The Songliao Basin of Northeast China preserves a near-complete, predominantly lacustrine, Cretaceous succession, with sedimentary cyclicity that has been tied to Milankocitch forcing of the climate. Over 900 meters of drill-core recovered from the Upper Cretaceous (Turonian to Campanian) of the Songliao Basin has provided a unique opportunity for detailed analyses of its depositional and paleoenvironmental records through integrated and high-resolution cyclostratigraphic, magnetostratigraphic and geochronologic investigations. Here we report high-precision U-Pb zircon dates (CA-ID-TIMS method) from four interbedded bentonites from the drill-core that offer substantial improvements in accuracy, and a ten-fold enhancement in precision, compared to the previous U-Pb SIMS geochronology, and allow a critical evaluation of the Songliao astrochronological time scale. The results indicate appreciable deviations of the astrochronologic model from the absolute radioisotope geochronology, which more likely reflect cyclostratigraphic tuning inaccuracies and omitted cycles due to depositional hiatuses, rather than suspected limitations of astronomical models applied to distant geologic time. Age interpolation based on our new high-resolution geochronologic framework and the calibrated cyclostratigraphy places the end of the Cretaceous Normal Superchon (C34n-C33r chron boundary) in the Songliao Basin at 83.07 ± 0.15 Ma. This date also serves as a new and improved estimate for the global Santonian-Campanian stage boundary.

Ancient Pb and Ti mobilization revealed by Scanning Ion Imaging

NASA Astrophysics Data System (ADS)

Kusiak, Monika A.; Whitehouse, Martin J.; Wilde, Simon A.

2014-05-01

Zircons from strongly layered early Archean ortho- and paragneisses in ultra-high temperature (UHT) metamorphic rocks of the Napier Complex, Enderby Land, East Antarctica are characterized by complex U-Th-Pb systematics [1,2,3]. A large number of zircons from three samples, Gage Ridge, Mount Sones and Dallwitz Nunatak, are reversely discordant (U/Pb ages older than 207Pb/206Pb ages) with the oldest date of 3.9 Ga [4] (for the grain from Gage Ridge orthogneiss). To further investigate this process, we utilized a novel high spatial resolution Scanning Ion Imaging technique on the CAMECA IMS 1280 at the Natural History Museum in Stockholm. Areas of 70 μm x 70 μm were selected for imaging in mono- and multicollection modes using a ~2 μm rastered primary beam to map out the distribution of 48Ti, 89Y, 180Hf, 232Th, 238U, 204Pb, 206Pb and 207Pb. The ion maps reveal variable distribution of certain elements within analysed grains that can be compared to their CL response. Yttrium, together with U and Th, exhibits zonation visible on the CL images, Hf shows expected minimal variation. Unusual patchiness is visible in the map for Ti and Pb distribution. The bright patches with enhanced signal do not correspond to any zones or to crystal imperfections (e.g. cracks). The presence of patchy titanium is likely to affect Ti-in-zircon thermometry, and patchy Pb affecting 207Pb/206Pb ages, usually considered as more robust for Archean zircons. Using the WinImage program, we produced 207Pb/206Pb ratio maps that allow calculation of 207Pb/206Pb ages for spots of any size within the frame of the picture and at any time after data collection. This provides a new and unique method for obtaining age information from zircon. These maps show areas of enhanced brightness where the 207Pb/206Pb ratio is higher and demonstrate that within these small areas (μm scale) the apparent 207Pb/206Pb age is older, in some of these patches even > 4 Ga. These data are a result of ancient Pb mobilization, which is independent of the degree of metamictisation, oxygen isotope and REE content of the zircons [5]. The Antarctic zircons experienced ancient Ti and Pb mobilization and redistribution, most likely caused by polyphase metamorphism at ~2.8 Ga and ~2.5 Ga, the latter documented as reaching temperatures of >1100ºC [5]. References: [1] Williams et al., 1984, Contr. Min. Petrol. 88, 322-327. [2] Black et al., (1986), Contr. Min. Petrol. 94, 427-437. [3] Harley & Kelly, 2007, Earth's Oldest Rocks: Developments in Precambrian Geology 15, 149-186. [4] Kusiak et al., 2013, Geology 41, 291-294. [5] Kusiak et al., 2013, American J. of Sci. 313, 933-967. [5] Hokada et al., 2004, Contr. Min. Petrol. 147, 1-20.

Timing of porphyry (Cu-Mo) and base metal (Zn-Pb-Ag-Cu) mineralisation in a magmatic-hydrothermal system—Morococha district, Peru

NASA Astrophysics Data System (ADS)

Catchpole, Honza; Kouzmanov, Kalin; Bendezú, Aldo; Ovtcharova, Maria; Spikings, Richard; Stein, Holly; Fontboté, Lluís

2015-12-01

The Morococha district in central Peru is characterised by economically important Cordilleran polymetallic (Zn-Pb-Ag-Cu) vein and replacement bodies and the large Toromocho porphyry Cu-Mo deposit in its centre. U-Pb, Re-Os, and 40Ar/39Ar geochronology data for various porphyry-related hydrothermal mineralisation styles record a 3.5-Ma multi-stage history of magmatic-hydrothermal activity in the district. In the late Miocene, three individual magmatic-hydrothermal centres were active: the Codiciada, Toromocho, and Ticlio centres, each separated in time and space. The Codiciada centre is the oldest magmatic-hydrothermal system in the district and consists of a composite porphyry stock associated with anhydrous skarn and quartz-molybdenite veins. The hydrothermal events are recorded by a titanite U-Pb age at 9.3 ± 0.2 Ma and a molybdenite Re-Os age at 9.26 ± 0.03 Ma. These ages are indistinguishable from zircon U-Pb ages for porphyry intrusions of the composite stock and indicate a time span of 0.2 Ma for magmatic-hydrothermal activity. The small Ticlio magmatic-hydrothermal centre in the west of the district has a maximum duration of 0.3 Ma, ranging from porphyry emplacement to porphyry mineralisation at 8.04 ± 0.14 Ma (40Ar/39Ar muscovite cooling age). The Toromocho magmatic-hydrothermal centre has a minimum of five recorded porphyry intrusions that span a total of 1.3 Ma and is responsible for the formation of the giant Toromocho Cu-Mo deposit. At least two hydrothermal pulses are identified. Post-dating a first pulse of molybdenite mineralisation, wide-spread hydrous skarn covers an area of over 6 km2 and is recorded by five 40Ar/39Ar cooling ages at 7.2-6.8 Ma. These ages mark the end of the slowly cooling and long-lived Toromocho magmatic-hydrothermal centre soon after last magmatic activity at 7.26 ± 0.02 Ma. District-wide (50 km2) Cordilleran base metal vein and replacement bodies post-date the youngest recorded porphyry mineralisation event at Toromocho by more than 0.5 Ma. Polymetallic veins (5.78 ± 0.10 and 5.72 ± 0.18 Ma; 40Ar/39Ar ages) and the Manto Italia polymetallic replacement bodies (6.23 ± 0.12 and 6.0 ± 0.2 Ma; 40Ar/39Ar ages) are interpreted to have been formed by a single hydrothermal pulse. Hydrothermal activity ceased after the formation of the base metal vein and replacement bodies. Overlapping monazite U-Pb (8.26 ± 0.18 Ma) and muscovite 40Ar/39Ar ages (8.1 ± 0.5 Ma) from the early base metal stage of one Cordilleran vein sample in the Sulfurosa area provide evidence that a discrete hydrothermal pulse was responsible for polymetallic vein formation 2.6 Ma prior to the district-wide polymetallic veins. These ages pre-date those of Toromocho porphyry Cu-Mo formation and show that Zn-Pb-Ag-Cu mineralisation formed during several discrete magmatic-hydrothermal pulses in the same district.

Rb-Sr, Sm-Nd, and U-Pb geochronology of the rocks within the Khlong Marui shear zone, southern Thailand

NASA Astrophysics Data System (ADS)

Kanjanapayont, Pitsanupong; Klötzli, Urs; Thöni, Martin; Grasemann, Bernhard; Edwards, Michael A.

2012-08-01

In southern Thailand, the Khlong Marui shear zone is dominated by a NNE-SSW striking high topographic lozenge shaped area of ca. 40 km long and 6 km wide between the Khlong Marui Fault and the Bang Kram Fault. The geology within this strike-slip zone consists of strongly deformed layers of mylonitic meta-sedimentary rocks associated with orthogneisses, mylonitic granites, and pegmatitic veins with a steeply dipping foliation. The strike-slip deformation is characterized by dextral ductile deformation under amphibolite facies and low to medium greenschist facies. In situ U-Pb ages of inherited zircon cores from all zircons in the Khlong Marui shear zone indicate that they have the same material from the Archean. Late Triassic to Late Cretaceous ages obtained for zircon outer cores of the mylonitic granite are probably related to a period of magmatic activity that was significantly influenced by the West Burma and Shan-Thai collision and the subduction along the Sunda Trench. The early dextral ductile deformation phase of the Khlong Marui shear zone in the Early Eocene suggested by U-Pb ages of zircon rims, and the later dextral transpressional deformation in the Late Eocene indicated by mica Rb-Sr ages. Rb-Sr, Sm-Nd, and U-Pb dating correlation implies that the major exhumation period of the ductile lens was in the Eocene. This period was tectonically influenced in the SE Asia region by the early India-Asia collision.

Dating sub-20 micron zircons in granulite-facies mafic dikes from SW Montana: a new approach using automated mineralogy and SIMS U-Pb geochronology

NASA Astrophysics Data System (ADS)

Ault, A. K.; Mahan, K. H.; Flowers, R. M.; Chamberlain, K.; Appleby, S. K.; Schmitt, A. K.

2010-12-01

Geochronological data is fundamental to all tectonic studies, but a major limitation for many lithologies is a paucity of sizeable zircons suitable for conventional U-Pb techniques. In particular, mafic dike swarms provide important time markers for tectonometamorphic activity in Precambrian terranes, but commonly yield little or no zircon or baddeleyite sufficient for TIMS or standard ion-probe analysis of crystal separates. We apply a new approach involving in-situ automated mineralogy and high spatial resolution Secondary Ion Mass Spectrometry (SIMS) geochronology to a mafic dike swarm exposed in the Northern Madison Range of SW Montana. The dikes cross-cut early fabrics but are also variably deformed and metamorphosed to P-T conditions as high as 1.2 GPa and 850 C. The swarm emplacement age is inferred to be ca. 2.1 Ga based on similarities to dated dikes in the adjacent Tobacco Root Mountains. Resolving the timing of dike emplacement and high-grade metamorphism in the study area is important for understanding the extent of post-Archean modification to the northwest margin of the Wyoming craton. Identification and textural characterization of zircons were facilitated by in-situ automated mineralogical analysis, in contrast to a standard elemental X-ray mapping approach. Our technique uses an SEM-based platform coupling calibrated BSE data with X-ray data collected by multiple energy dispersive spectrometers to rapidly identify target accessory phases at high spatial resolution. Whole thin section search maps were generated in ~30 minutes at 4 µm pixel resolution. Our dike thin sections commonly contained >300 zircons in a variety of textural settings, with 80% having a short dimension <10 µm. Zircons were dated in-situ by adjusting the field aperture of the CAMECA ims1270 to preferentially collect secondary ions emitted from within the inner few microns of the ~15 µm diameter analysis pit. This allows us to analyze zircon grains with a minimum dimension as small as 8 μm at radiogenic yields typically >95% for 206Pb. SIMS data for 22 zircons from a granulite-facies mafic dike thin section define a chord with upper and lower intercepts of 1753.1 ± 9.5 Ma and 63.2 ± 7.9 Ma, respectively (2 sigma error, MSWD = 1.6). A positive correlation between U concentration and degree of discordance indicates that the more radiation-damaged zircons underwent greater Pb loss. We infer Pb loss to reflect re-heating linked to emplacement of the nearby Tobacco Roots Batholith (ca. 74-71 Ma). Metamorphic zircon growth ca. 1750 Ma indicates that the high-grade metamorphic core of the Big Sky orogeny extends into the Northern Madison Range, farther inboard into the Wyoming craton than previously recognized. Coupling automated mineralogy searching with refined SIMS methods enables acquisition and interpretation of in-situ U-Pb data from zircons of a size that would not be feasible with most other techniques.

U-Th-Pb zircon ages of some Keweenawan Supergroup rocks from the south shore of Lake Superior

USGS Publications Warehouse

Zartman, R.E.; Nicholson, S.W.; Cannon, W.F.; Morey, G.B.

1997-01-01

New single-crystal zircon U-Th-Pb ages for plutonic and rhyolitic Keweenawan Supergroup rocks from the south shore of Lake Superior provide geochronological constraints on magmatic evolution associated with the 1.1 Ga Midcontinent rift. Analyses of a granophyric phase of the Mineral Lake intrusion and the Meilen granite, both parts of the Meilen Intrusive Complex, and a laterally extensive rhyolite from the top of the Kallander Creek Volcanics have weighted average 207Pb/206Pb ages of 1102.0 ?? 2.8 Ma (N = 2), 1100.9 ?? 1.4 Ma (N = 5), and 1098.8 ?? 1.9 Ma (N = 4), respectively. Analyses of a pyroclastic rhyolite flow at the top of the Porcupine Volcanics result in variable 207Pb/206Pb ages that range from 1080 to 1137 Ma. This rhyolite exhibits a continuum between morphologically complex and simpler prismatic zircon crystals, the latter yielding concordant analyses having a weighted average 207Pb/206Pb age of 1093.6 ?? 1.8 Ma (N = 2). Four prismatic zircons from an aphyric rhyolite of the Chengwatana Volcanics in the Ashland syncline form a linear array intersecting concordia at 1094.6 ?? 2.1 Ma (MSWD = 1.3). Another presumed Chengwatana rhyolite recovered from drill core intersecting the Hudson-Afton horst in southeast Minnesota yielded only ???20 morphologically indistinguishable zircons. Six analyses give 207Pb/206Pb ages ranging from 1112 to 1136 Ma, including one analysis with a virtually concordant age of 1130 Ma. This age, however, is considerably older than that obtained for the Chengwatana Volcanics in the Ashland syncline or any other precisely dated rock from the Midcontinent rift.

The Age of the Moon

NASA Astrophysics Data System (ADS)

Barboni, M.; Boehnke, P.; Keller, C. B.; Kohl, I. E.; McKeegan, K. D.; Schoene, B.; Young, E. D.

2016-12-01

Knowledge of the age of the Moon is important for understanding the early evolution of the solar system, including the timing of the hypothesized Giant Impact (GI). There have been many attempts to determine the Moon's age, but significant disagreement remains with some authors favoring an early formation and others arguing for a relatively young Moon formed at 4.4 Ga. Attempts to date the GI indirectly through its effects on the asteroid belt are problematic as there is no way to uniquely ascertain the cause of the observed disturbances (e.g., GI or meteorite parent body breakup). Determining the timing of the Lunar Magma Ocean (LMO) crystallization provides a more direct constraint on the age of the Moon, but interpreting the chronologic significance of LMO products is complicated by the fact that the only rock samples available are breccias. A better approach is to construct a model age for the fractional crystallization of the LMO since this should provide a global signature. Zircons from the Apollo samples are ancient, robust against later disturbances, and amenable to precise U-Pb geochronology and Hf isotope analyses that can be used to construct Lu-Hf model ages for the silicate differentiation of the Moon. Previous isotopic studies of Apollo zircons yielded artificially young Hf model ages because of the (then unknown) effect of neutron capture on Hf isotopic ratios generated by long exposure to cosmic radiation, and were unable to determine whether or not the U-Pb dates were concordant due to insufficient precision of in situ dating techniques. We have addressed these issues by carrying out CA-ID-TIMS U-Pb geochronology on Apollo 14 zircon fragments, followed by Hf isotope determination by solution MC-ICP-MS on the same volume of zircon. By constructing Hf model ages from zircons that are concordant to the sub-permil level, we show that the minimum age for the end of differentiation of the LMO, and by extension, the formation of the Moon, is 4.52 ± 0.01 Ga.

Concordant Rb-Sr and Sm-Nd Ages for NWA 1460: A 340 Ma Old Basaltic Shergottite Related to Lherzolitic Shergottites

NASA Technical Reports Server (NTRS)

Nyquist, L. E.; Shih, C-Y; Reese, Y. D.; Irving, A. J.

2006-01-01

Preliminary Rb-Sr and Sm-Nd ages reported by [1] for the NWA 1460 basaltic shergottite are refined to 336+/-14 Ma and 345+/-21 Ma, respectively. These concordant ages are interpreted as dating a lava flow on the Martian surface. The initial Sr and Nd isotopic compositions of NWA 1460 suggest it is an earlier melting product of a Martian mantle source region similar to those of the lherzolitic shergottites and basaltic shergottite EETA79001, lithology B. We also examine the suggestion that generally "young" ages for other Martian meteorites should be reinterpreted in light of Pb-207/Pb-206 - Pb-204/Pb-206 isotopic systematics [2]. Published U-Pb isotopic data for nakhlites are consistent with ages of approx.1.36 Ga. The UPb isotopic systematics of some Martian shergottites and lherzolites that have been suggested to be approx.4 Ga old [2] are complex. We nevertheless suggest the data are consistent with crystallization ages of approx.173 Ma when variations in the composition of in situ initial Pb as well as extraneous Pb components are considered.

238U/235U determinations of some commonly used reference materials and U-bearing accessory minerals (Invited)

NASA Astrophysics Data System (ADS)

Condon, D.; Noble, S.; McLean, N.; Bowring, S. A.

2009-12-01

We have determined 238U/235U ratios for a suite of commonly used natural (CRM 112a, SRM 950a, HU-1) and synthetic (IRMM 184 and CRM U500) uranium reference materials in addition to several U-bearing accessory phases (zircon and monazite) by thermal ionisation mass-spectrometry (TIMS) using the IRMM 3636 233U-236U double spike to accurately correct for mass fractionation. The 238U/235U values for the natural uranium reference materials differ, by up to 0.1%, from the widely used ‘consensus’ value (137.88) with all having 238U/235U values less than 137.88. Similarly, initial 238U/235U data from zircon and monazite yield 238U/235U values that are lower than the ‘consensus’ value. The data obtained from U-bearing minerals is used to assess how the uncertainty in the 238U/235U ratio contributes to the systematic discordance observed in 238U/206Pb and 235U/207Pb dates (Mattinson, 2000; Schoene et al., 2006) which has traditionally been wholly attributed to error in the U decay constants. The 238U/235U determinations made on the synthetic reference materials yield results that are considerably more precise and accurate than the certified values (0.02% vs. 0.1% for CRM U500). The calibration of isotopic tracers used for U-daughter geochronology that are partially based upon these reference materials, and the resultant age determinations, will benefit from increased accuracy and precision. Mattinson, J.M., 2000. Revising the “gold standard”—the uranium decay constants of Jaffey et al., 1971. Eos Trans. AGU, Spring Meet. Suppl., Abstract V61A-02. Schoene B., Crowley J.L., Condon D.C., Schmitz M.D., Bowring S.A., 2006, Reassessing the uranium decay constants for geochronology using ID-TIMS U-Pb data. Geochimica et Cosmochimica Acta 70: 426-445

Cretaceous crust beneath SW Borneo: U-Pb dating of zircons from metamorphic and granitic rocks

NASA Astrophysics Data System (ADS)

Davies, L.; Hall, R.; Armstrong, R.

2012-12-01

Metamorphic basement rocks from SW Borneo are undated but have been suggested to be Palaeozoic. This study shows they record low pressure 'Buchan-type' metamorphism and U-Pb SHRIMP dating of zircons indicates a mid-Cretaceous (volcaniclastic) protolith. SW Borneo is the southeast promontory of Sundaland, the continental core of SE Asia. It has no sedimentary cover and the exposed basement has been widely assumed to be a crustal fragment from the Indochina-China margin. Metamorphic rocks of the Pinoh Group in Kalimantan (Indonesian Borneo) are intruded by granitoid rocks of Jurassic-Cretaceous age, based on K-Ar dating, suggesting emplacement mainly between 130 and 80 Ma. The Pinoh metamorphic rocks have been described as a suite of pelitic schists, slates, phyllites, and hornfelses, and have not been dated, although they have been correlated with rocks elsewhere in Borneo of supposed Palaeozoic age. Pelitic schists contain biotite, chlorite, cordierite, andalusite, quartz, plagioclase and in some cases high-Mn almandine-rich garnet. Many have a shear fabric associated with biotite and fibrolite intergrowth. Contact metamorphism due to intrusion of the granitoid rocks produced hornfelses with abundant andalusite and cordierite porphyroblasts. Granitoids range from alkali-granite to tonalite and contain abundant hornblende and biotite, with rare white mica. Zircons from granitoid rocks exhibit sector- and concentric- zoning; some have xenocrystic cores mantled by magmatic zircon. There are four important age populations at c. 112, 98, 84 and 84 Ma broadly confirming earlier dating studies. There is a single granite body with a Jurassic age (186 ± 2.3 Ma). Zircons from pelitic metamorphic rocks are typically euhedral, with no evidence of rounding or resorbing of grains; a few preserve volcanic textures. They record older ages than those from igneous rocks; U-Pb ages are Cretaceous with a major population between 134 and 110 Ma. A single sample contains Proterozoic and Phanerozoic zircons. The metamorphic rocks from SW Borneo are not an ancient core to the island as previously assumed. We propose that extensive arc volcanism produced fine grained volcanogenic sediments during the Early Cretaceous deposited on, or reworking, older crust. These sediments were subjected to low pressure 'Buchan-type' metamorphism soon after deposition. Magmatism continued into the Late Cretaceous, resulting in contact metamorphism.

Standard Materials for Microbeam Analysis of Lanthanides and Actinides

NASA Astrophysics Data System (ADS)

Ellis, I.; Gorton, M.; Rucklidge, J. C.

2010-12-01

Traces of Th and U in naturally-occuring minerals monazite, xenotime and zircon are used for dating host rocks. Natural variations of actinide concentrations in some rock formations are well documented. Microbeam techniques perform dating in-situ where grains of indicator minerals are left intact in thin sections. Separated individual grains of these minerals are also routinely dated by Pb-isotope mass spectrometry. Ideal calibration materials will be compatible with multiple techniques. Quantitative analysis of low levels of lanthanides (REE), U, Th and Pb found in natural minerals requires standards containing similar concentrations of these elements. The ideal low-level standard suite will have materials with each REE cation present below 5%, similar to natural rare-earth phosphate minerals. In contrast, REE orthophosphates LnPO4 have cation concentrations from 59 to 64%, and ultraphosphates LnP5O14 from 27% to 32%. The concentrations of U and Pb must also be in the 1% range in the host REE phosphate. There are two competing limits to the synthesis of crystals with multiple cations in the REE sites. The crystal structure limits potential cation mixtures to selections within groups (La,Ce, Pr, Nd, Sm, Eu), (Gd, Tb, Dy, Ho), and (Er, Tm,Yb, Lu, Y). Complex L X-ray spectra limit the use of contiguous REE in a single material. There are two general synthetic routes for the preparation of lanthanide/actinide standard materials for beam analysis and dating. Lanthanide orthophosphates (LnPO4) are crystallized from lead-free heterogeneous fluxes; oligomers (metaphosphates LnP3O9 and ultraphosphates LnP5O14) are formed by condensation of phosphoric acid in the presence of cations. All of these trivalent lanthanide phosphate crystal structures are hosts for Th+4 and U+4, and in synthetic materals, Ca+2 is used for charge compensation. Our work focuses on the preparation of mixed-cation lanthanide metaphosphates and ultraphosphates. The solvent (essentially P2O5) provides redox conditions that favour Ce+3, Th+4, and U+4 instead of higher oxidation states. The absence of any cations other than those deliberately added permits positive control of cation mixtures in starting materials. The synthetic pathway—condensation of POx units--provides ideal conditions for the homogeneous distribution of cations including those with different charges. We present the results of synthesis, elemental analysis and imaging by XRF and SEM-EDX for mixed lanthanide-actinide phosphate materials.

Ar-Ar and U-Pb ages of marble-hosted ruby deposits from Central and South-east Asia

NASA Astrophysics Data System (ADS)

Garnier, V.; Giuliani, G.; Maluski, H.; Ohnenstetter, D.; Deloule, E.

2003-04-01

Marble-hosted ruby deposits represent the first source of gemstones in Asia. The deposits from Jegdalek (Afghanistan), Hunza Valley (Pakistan), Nangimali (Azad-Kashmir), Chumar, Ruyil (Nepal), Mogok (Myanmar), Luc Yen, Yen Bai and Quy Chau (Vietnam) were dated using the 40Ar-39Ar laser stepwise heating technique on syngenetic micas. The following ages were obtained : 24.7 ± 0.3 Ma at Jegdalek ; 10.8 ± 0.3 to 5.4 ± 0.3 Ma at Hunza ; 17.2 ± 0.2 to 15.3 ± 0.1 Ma at Nangimali ; 4.6 ± 0.1 Ma at Ruyil ; 5.6 ± 0.4 Ma at Chumar ; 18.7 ± 0.2 to 17.1 ± 0.2 Ma at Mogok ; 33.8 ± 0.4 to 30.8 ± 0.8 Ma at Luc Yen ; 24.4 ± 0.4 to 23.2 ± 0.6 Ma at Yen Bai, 22.1 ± 0.6 to 21.6 ± 0.7 Ma at Quy Chau. These ages represent cooling ages and thus minimum ages for ruby formation. The ages obtained for Nangimali are close to the Ar-Ar cooling age of 19 Ma recorded in the Chichi granite, North to the ruby deposit. However, (C,O)-isotopic studies of the ruby-bearing marbles show no genetic relation between granite emplacement and ruby deposition in this area. The age found at Jegdalek is similar to the K-Ar ages obtained on the Sairobi pegmatitic dykes (20-26 Ma) and of the Jalalabad pluton (25 Ma), located close to the ruby deposit. At Mogok, the ruby deposits yield ages close to those obtained on high grade metamorphic and foliated intrusive regional rocks (15.8 ± 0.7 - 19.5 ± 1.0 Ma). The ages obtained at Chumar and Ruyil agree with those of the Lesser Himalaya Formation (12 - 6 Ma). Those found at Quy Chau agree with those found for the shear zone activity. Furthermore, U-Pb dating was done on zircons included in a ruby from Luc Yen and spinels in marble from Luc Yen and Hunza. The wide range of 238U-206Pb ages obtained for Luc Yen (266 - 45 Ma) evidences a complex metamorphic history. Ruby crystallised at 45 Ma during ductile activity of the Red River shear zone. At Hunza, an 238U-206Pb age of 94.0 ± 2.1 Ma obtained on inherited zircons confirms the U-Pb age obtained on zircons from the Karakorum batholith (95 Ma). Asian marble-hosted ruby deposits are directly linked with the tectonometamorphic activity of Cenozoic structures resulting from deformation of the Asian plate during India-Asia collision. Geochemical studies evidence that these rubies have a metamorphic origin and thus dating of these deposits provide an essential clue to decipher the timing of continental collision in Central and South-east Asia.

Combined garnet and zircon geochronology and trace elements studies - constraints of the UHP-(U)HT evolution of Orlica-Śnieżnik Dome (NE Bohemian Massif).

NASA Astrophysics Data System (ADS)

Walczak, Katarzyna; Anczkiewicz, Robert; Szczepański, Jacek; Rubatto, Daniela

2017-04-01

The Orlica-Śnieżnik Dome (OSD), located on the NE margin of the Bohemian Massif, is predominantly composed of amphibolite-facies orthogneiss that contain bodies of HP and UHP eclogites and granulites. Numerous geochronological studies have been undertaken to constrain the timing of the ultra-high grade metamorphic event. Despite this, the exact timing of UHP-(U)HT conditions remain dubious (e.g. Brueckner et al., 1991; Anczkiewicz et al., 2007; Bröcker et al., 2009 & 2010). We have utilized garnet and zircon geochronology to provide time constraints on the evolution of the UHT-(U)HP rocks of the OSD. We have combined the ages with trace element analyses in garnet and zircon to better understand the significance of the obtained ages in petrological context. Lu-Hf grt-wr dating of peritectic garnet from two felsic granulites constrained the time of its initial growth at 346.9 ± 1.2 and 348.3 ± 2.0 Ma, recording peak conditions of 2.7 GPa and 950°C (e.g. Ferrero et al., 2015). In situ U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 342.2 ± 3.4 Ma. HREE partitioning between garnet rim and metamorphic zircon indicate their growth in equilibrium, hence, the U-Pb zircon date constrains the terminal phase of garnet crystallization. Similar ages were obtained from two eclogite bodies from Międzygórze and Nowa Wieś localities; Lu-Hf (grt-cpx-wr) dating provided ages of 346.5 ± 2.4 and 348.1 ± 9.1 Ma for samples from Międzygórze and Nowa Wieś, respectively. The same age (within error) of 346.3 ± 5.2 Ma was reported by Bröcker et al. (2010) for zircon from the Międzygórze eclogite. Comparison of REE concentrations in garnet (this study) and in metamorphic zircon (reported in Bröcker et al., 2010) indicate that garnet and zircon crystallized in equilibrium. Furthermore, M-HREE patterns observed in both garnet and zircon strongly suggest their growth at eclogite facies conditions. Sm-Nd garnet ages obtained for both felsic and mafic granulites and eclogites are identical within error and are consistently younger than corresponding Lu-Hf dates. Sm-Nd grt-wr ages of two samples of felsic granulite provide 332.4 ± 5.2 and 337.6 ± 2.3 Ma, while Sm-Nd grt-cpx age of a mafic granulite provides 336.9 ± 6.0 Ma. Sm-Nd grt-cpx(-wr) ages obtained for three eclogite samples range from 336.2 ± 3.5 to 337.7 ± 2.6 Ma. The foregoing ages are interpreted to reflect cooling through the Sm-Nd closure temperature at about 337 Ma. The estimated PTt path documents the evolution of the OSD, characterized by two distinct periods: (1) nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites at 347-342 Ma, and (2) fast, nearly isobaric cooling at 342 - 337 Ma, becoming very rapid towards the end of this period. Anczkiewicz, R. et al. 2007. Lithos, 95, 363-380. Bröcker , M. et al. 2009. Journal of Metamorphic Geology, 27, 385-403. Bröcker, M. et al. 2010. Geological Magazine, 147(3), 339-362. Brueckner, H. K. et al. 1991. Neues Jahrb Mineral Abh, 63, 169-193. Ferrero S. et al. 2015. Geology, 43, 447-450.

Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages

NASA Astrophysics Data System (ADS)

Erickson, Timmons M.; Timms, Nicholas E.; Kirkland, Christopher L.; Tohver, Eric; Cavosie, Aaron J.; Pearce, Mark A.; Reddy, Steven M.

2017-03-01

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U-Th-Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), ( 10bar{1} ), {110}, { 212 }, and type two (irrational) twin planes with rational shear directions in [0bar{1}bar{1}] and [bar{1}bar{1}0]. SIMS U-Th-Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a 207Pb/206Pb age of 2010 ± 15 Ma (2 σ, n = 9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259 ± 5 Ma (2 σ, n = 7), which is consistent with previous impact age estimates of 255 ± 3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U-Th-Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts.

Towards Synthesis and Usage of Actinide-Bearing REE Phosphate age Standards: A Progress Report

NASA Astrophysics Data System (ADS)

Pyle, J. M.; Cherniak, D. J.

2006-05-01

Electron microprobe (EMP) dates result from a concentration-time unit conversion, so use of a concentration- based (rather than isotope-ratio based) fictive age standard is warranted. This observation has motivated our mineral synthesis program, aimed at producing actinide-doped REE phosphate EMP dating standards that meet the following criteria: 1) known concentrations of U, Th, and Pb; 2) homogeneous intragrain distribution of all components; 3) of suitable size, either as a single-crystal or polycrystalline sintered ceramic. Single-crystal synthesis of actinide-doped LaPO4 by flux-growth methods results in disproportionation of lanthanide and flux, alkali, and actinide components into phosphate and oxide phases, respectively, and flux- growth methods were abandoned. Actinide-doped La phosphate is successfully prepared by high-T annealing and hydrothermal processing of microcrystalline phosphate; both homogeneity and charge-balance of (Ca, Th, Pb)-bearing LaPO4 increase with increasing solvent acidity during cold-seal hydrothermal synthesis. A combination of pressing and high-T (1400° C) sintering transforms fine-grained (0.1-10 μm) run- products to ceramic pellets with 90-95% theoretical density. Our most recent runs focused on a target composition of La80(CaTh)17(CaU)2(PbTh)1PO4 processed with 6% 2M HCl at 820° C, 0.75 kbar for 1 week. The run products are 0.1-2 μm crystals identified by XRD as La-actinide phosphate solid solution. 2 μm grains (N=16) give a composition (mean±2 sd) of La79.77(1.26)(CaTh)17.87(1.00)(CaU)1.53(0.42)(PbTh)0.82(0.09)PO4. Th (8.07-9.13 wt. %) is homogeneous at the level of analytical precision, and the Pb concentration range (3500-4350 ppm) is restricted relative to untreated precipitate. Uranium concentration values are more variable (6500-10000 ppm). This run yields a fictive age of 702±4 Ma (mean±2 se), compared to the fictive age of 794 Ma for the target composition.

The basement of the Mount Athos peninsula, northern Greece: insights from geochemistry and zircon ages

NASA Astrophysics Data System (ADS)

Himmerkus, F.; Zachariadis, P.; Reischmann, T.; Kostopoulos, D.

2012-09-01

The Mount Athos Peninsula is situated in the south-easternmost part of the Chalkidiki Peninsula in northern Greece. It belongs to the Serbo-Macedonian Massif (SMM), a large basement massif within the Internal Hellenides. The south-eastern part of the Mount Athos peninsula is built by fine-grained banded biotite gneisses and migmatites forming a domal structure. The southern tip of the peninsula, which also comprises Mount Athos itself, is built by limestone, marble and low-grade metamorphic rocks of the Chortiatis Unit. The northern part and the majority of the western shore of the Mount Athos peninsula are composed of highly deformed rocks belonging to a tectonic mélange termed the Athos-Volvi-Suture Zone (AVZ), which separates two major basement units: the Vertiskos Terrane in the west and the Kerdillion Unit in the east. The rock-types in this mélange range from metasediments, marbles and gneisses to amphibolites, eclogites and peridotites. The gneisses are tectonic slivers of the adjacent basement complexes. The mélange zone and the gneisses were intruded by granites (Ierissos, Ouranoupolis and Gregoriou). The Ouranoupolis intrusion obscures the contact between the mélange and the gneisses. The granites are only slightly deformed and therefore postdate the accretionary event that assembled the units and created the mélange. Pb-Pb- and U-Pb-SHRIMP-dating of igneous zircons of the gneisses and granites of the eastern Athos peninsula in conjunction with geochemical and isotopic analyses are used to put Athos into the context of a regional tectonic model. The ages form three clusters: The basement age is indicated by two samples that yielded Permo-Carboniferous U-Pb-ages of 292.6 ± 2.9 Ma and 299.4 ± 3.5 Ma. The main magmatic event of the granitoids now forming the gneiss dome is dated by Pb-Pb-ages between 140.0 ± 2.6 Ma and 155.7 ± 5.1 Ma with a mean of 144.7 ± 2.4 Ma. A within-error identical age of 146.6 ± 2.3 Ma was obtained by the U-Pb-SHRIMP method. This Late Jurassic age is also known from the Kerdillion Unit and the Rhodope Terrane. The rather undeformed granites are interpreted as piercing plutons. The small granite stocks sampled have Late Cretaceous to Early Tertiary ages of 66.8 ± 0.8 Ma and 68.0 ± 1.0 Ma (U-Pb-SHRIMP)/62.8 ± 3.9 Ma (Pb-Pb). The main accretionary event was according to these data in the Late Jurassic since all younger rocks show little or no deformation. The age distribution together with the geochemical and isotopic signature and the lithology indicates that the eastern part of the Mount Athos peninsula is part of a large-scale gneiss dome also building the Kerdillion Unit of the eastern SMM and the Rhodope Massif. This finding extends the area of this dome significantly to the south and indicates that the tectonic boundary between the SMM and the Rhodope Massif lies within the AVZ.

Cellulose nanofiber backboned Prussian blue nanoparticles as powerful adsorbents for the selective elimination of radioactive cesium.

PubMed

Vipin, Adavan Kiliyankil; Fugetsu, Bunshi; Sakata, Ichiro; Isogai, Akira; Endo, Morinobu; Li, Mingda; Dresselhaus, Mildred S

2016-11-15

On 11 March 2011, the day of the unforgettable disaster of the 9 magnitude Tohoku earthquake and quickly followed by the devastating Tsunami, a damageable amount of radionuclides had dispersed from the Fukushima Daiichi's damaged nuclear reactors. Decontamination of the dispersed radionuclides from seawater and soil, due to the huge amounts of coexisting ions with competitive functionalities, has been the topmost difficulty. Ferric hexacyanoferrate, also known as Prussian blue (PB), has been the most powerful material for selectively trapping the radioactive cesium ions; its high tendency to form stable colloids in water, however, has made PB to be impossible for the open-field radioactive cesium decontamination applications. A nano/nano combinatorial approach, as is described in this study, has provided an ultimate solution to this intrinsic colloid formation difficulty of PB. Cellulose nanofibers (CNF) were used to immobilize PB via the creation of CNF-backboned PB. The CNF-backboned PB (CNF/PB) was found to be highly tolerant to water and moreover, it gave a 139 mg/g capability and a million (10 6 ) order of magnitude distribution coefficient (K d ) for absorbing of the radioactive cesium ion. Field studies on soil and seawater decontaminations in Fukushima gave satisfactory results, demonstrating high capabilities of CNF/PB for practical applications.

Cellulose nanofiber backboned Prussian blue nanoparticles as powerful adsorbents for the selective elimination of radioactive cesium

PubMed Central

Vipin, Adavan Kiliyankil; Fugetsu, Bunshi; Sakata, Ichiro; Isogai, Akira; Endo, Morinobu; Li, Mingda; Dresselhaus, Mildred S.

2016-01-01

On 11 March 2011, the day of the unforgettable disaster of the 9 magnitude Tohoku earthquake and quickly followed by the devastating Tsunami, a damageable amount of radionuclides had dispersed from the Fukushima Daiichi’s damaged nuclear reactors. Decontamination of the dispersed radionuclides from seawater and soil, due to the huge amounts of coexisting ions with competitive functionalities, has been the topmost difficulty. Ferric hexacyanoferrate, also known as Prussian blue (PB), has been the most powerful material for selectively trapping the radioactive cesium ions; its high tendency to form stable colloids in water, however, has made PB to be impossible for the open-field radioactive cesium decontamination applications. A nano/nano combinatorial approach, as is described in this study, has provided an ultimate solution to this intrinsic colloid formation difficulty of PB. Cellulose nanofibers (CNF) were used to immobilize PB via the creation of CNF-backboned PB. The CNF-backboned PB (CNF/PB) was found to be highly tolerant to water and moreover, it gave a 139 mg/g capability and a million (106) order of magnitude distribution coefficient (Kd) for absorbing of the radioactive cesium ion. Field studies on soil and seawater decontaminations in Fukushima gave satisfactory results, demonstrating high capabilities of CNF/PB for practical applications. PMID:27845441

New zircon U-Pb LA-ICP-MS ages and Hf isotope data from the Central Pontides (Turkey): Geological and geodynamic constraints

NASA Astrophysics Data System (ADS)

Çimen, Okay; Göncüoğlu, M. Cemal; Simonetti, Antonio; Sayit, Kaan

2018-05-01

The Central Pontides in northern Anatolia is located on the accretionary complex formed by the closure of Neotethyan Intra-Pontide Ocean between the southern Eurasian margin (Istanbul-Zonguldak Terrane) and the Cimmerian Sakarya Composite Terrane. Among other components of the oceanic lithosphere, it comprises not yet well-dated felsic igneous rocks formed in arc-basin as well as continent margin settings. In-situ U-Pb age results for zircons from the arc-basin system (öangaldağ Metamorphic Complex) and the continental arc (Devrekani Metadiorite and Granitoid) yield ages of 176 ± 6 Ma, 163 ± 9 Ma and 165 ± 3 Ma, respectively. Corresponding in-situ average (initial) 176Hf/177Hf initial ratios are 0.28261 ± 0.00003, 0.28267 ± 0.00002 and 0.28290 ± 0.00004 for these units and indicative of a subduction-modified mantle source. The new U-Pb ages and Hf isotope data from these oceanic and continental arc units together with regional geological constraints support the presence of a multiple subduction system within the Intra-Pontide Ocean during the Middle Jurassic.

Radiometric dates from Alaska: A 1975 compilation

USGS Publications Warehouse

Turner, D.L.; Grybeck, Donald; Wilson, Frederic H.

1975-01-01

The following table of radiometric dates from Alaska includes published material through 1972 as well as some selected later data. The table includes 726 mineral and whole-rock dates determined by the K-Ar, Rb-Sr, fission-track U-Pb, and Pb-alpha techniques.The data are organized in alphabetical order of the 1:250,000 scale quadrangles in which the dated rocks are located. The latitude and longitude of each sample are given. In addition, each sample is located on a 1:250,000 quadrangle map by a grid system. The initial point of the grid is taken as the southwest corner of the quadrangle and the location of the sample is measured in inches east and inches north from that corner, e.g., "156E 126N" indicated 15.6 inches east and 12.6 inches north of the southwest corner of the quadrangle. Zeroes in the location columns for some dates indicate that accurate locations are not available.Rock type, dating method, mineral dated, radiometric age, sample identification number, and reference are also listed where possible. Short comments, mostly geographic locality names, are given for some dates. These comments have been taken from the original references.Sample identification numbers beginning with "AA" or "BB" have been assigned arbitrarily in cases where sample numbers were not assigned in the original references. Abbreviations are explained in the appendix at the end of table 1.

Geochemical and zircon isotopic evidence for extensive high level crustal contamination in Miocene to mid-Pleistocene intra-plate volcanic rocks from the Tengchong field, western Yunnan, China

NASA Astrophysics Data System (ADS)

Li, Linlin; Shi, Yuruo; Williams, Ian S.; Anderson, J. Lawford; Wu, Zhonghai; Wang, Shubing

2017-08-01

SHRIMP zircon Pb/U dating of Cenozoic volcanic rocks in the Tengchong area, western Yunnan Province, China, shows that the dacite and andesitic breccia lavas from Qushi village were intruded at 480 ± 10 ka and 800 ± 40 ka, respectively. Moreover, Pb/U dating of trachyandesite from Tuantian village and olivine basalt from Wuhe village give weighted mean 206Pb/238U ages of 2.82 ± 0.08 Ma and 12.28 ± 0.30 Ma. Corrections for initial 230Th disequilibrium of zircon were used for the former two younger ages. The Tengchong volcanic rocks have a large range of SiO2 (48.6-66.9 wt.%) and mostly belong to a high-K calc-alkaline series. The lavas originated from heterogeneous sources and were modified by subsequent fractional crystallization. The REE and other trace element patterns of the Tengchong volcanic rocks resemble magmas having a large component of continental crust. All have similar degrees of LREE and HREE fractionation and are enriched in LILE, La, Ce and Pb, with depletions in Nb, Ta, Ti, Sr and P relative to primitive mantle. Zircon δ18O values of 6.96 ± 0.17 and 7.01 ± 0.24‰ and highly varied negative εHf(t) values of - 1.5 to - 11.0 and - 10.3 to - 13.7, as well as the presence of inherited zircon grains in the studied samples, indicate that the magmas contain crustal material on a large scale. The Tengchong volcanic rocks have HFSE ratios (e.g., Nb/Ta, La/Nb, Zr/Y) similar to continental flood basalts, indicative of an intra-plate extensional tectonic setting. Widespread distributed faults might have facilitated upwelling of mantle-derived melts and eruptions from shallow crustal magma chambers to form the large volcanic field.

U-Pb isotopic systematics of zircons from prograde and retrograde transition zones in high-grade orthogneisses, Sri Lanka

DOE Office of Scientific and Technical Information (OSTI.GOV)

Baur, N.; Liew, T.C.; Todt, W.

1991-07-01

The authors present U-Pb zircon isotopic data from locally restricted prograde (arrested in situ charnockitization) and retrograde metamorphic transition zones, which are well exposed in Proterozoic orthogneisses tectonically interbanded with granulite facies supracrustal rocks of the Highland Group in Sri Lanka. These granitoid rocks yield apparent ages of 1942 {plus minus} 22 Ma, {approximately} 770 Ma, {approximately} 660 Ma, and {approximately} 560 Ma. All samples show severe Pb-loss some 550-560 Ma ago. The main phase of granulite-formation could not be dated unambiguously but is bracketed between {approximately} 660 Ma and {approximately} 550 Ma. The pervasive Pb-loss event around 550-560 Mamore » reflects the end of this period of high-grade metamorphism and was associated with widespread igneous activity and retrogression. This is constrained by the 550 {plus minus} 3 Ma intrusion age for a post-tectonic granite. They relate this late phase of thermal activity to crustal uplift of the Sri Lankan granulites. This data unambiguously prove the high-grade history of the Sri Lanka gneisses to be a late Precambrian event that may be related to the Pan-African evolution along the eastern part of Africa.« less

Applications of detrital geochronology and thermochronology from glacial deposits to the Paleozoic and Mesozoic thermal history of the Ross Embayment, Antarctica

NASA Astrophysics Data System (ADS)

Welke, Bethany; Licht, Kathy; Hennessy, Andrea; Hemming, Sidney; Pierce Davis, Elizabeth; Kassab, Christine

2016-07-01

Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U-Pb age population spanning the time of the Ross/Pan-African Orogenies (610-475 Ma). Geochronology and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double-dating of the zircons reveals a geographically widespread (U-Th)/He (ZHe) population of 180-130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (>200 Ma) ZHe ages in tills at the head of Byrd, Nimrod, and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glaciers, integrating the double-dated zircon results with 40Ar/39Ar of hornblende, muscovite and biotite, and U-Pb and (U-Th-Sm)/He double-dates on apatite yields a typical pattern of early rapid orogenic cooling (˜4-10°C/Myr) 590-475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480-70 Ma and AHe 200-70 Ma) that require a long history at low temperature.

Mass-spectrometric mining of Hadean zircons by automated SHRIMP multi-collector and single-collector U/Pb zircon age dating: The first 100,000 grains

NASA Astrophysics Data System (ADS)

Holden, Peter; Lanc, Peter; Ireland, Trevor R.; Harrison, T. Mark; Foster, John J.; Bruce, Zane

2009-09-01

The identification and retrieval of a large population of ancient zircons (>4 Ga; Hadean) is of utmost priority if models of the early evolution of Earth are to be rigorously tested. We have developed a rapid and accurate U-Pb zircon age determination protocol utilizing a fully automated multi-collector ion microprobe, the ANU SHRIMP II, to screen and date these zircons. Unattended data acquisition relies on the calibration of a digitized sample map to the Sensitive High Resolution Ion MicroProbe (SHRIMP) sample-stage co-ordinate system. High precision positioning of individual grains can be produced through optical image processing of a specified mount location. The focal position of the mount can be optimized through a correlation between secondary-ion steering and the spot position on the target. For the Hadean zircon project, sample mounts are photographed and sample locations (normally grain centers) are determined off-line. The sample is loaded, reference points calibrated, and the target positions are then visited sequentially. In SHRIMP II multiple-collector mode, zircons are initially screened (ca. 5 s data acquisition) through their 204Pb corrected 207Pb/206Pb ratio; suitable candidates are then analyzed in a longer routine to obtain better measurement statistics, U/Pb, and concentration data. In SHRIMP I and SHRIMP RG, we have incorporated the automated analysis protocol to single-collector measurements. These routines have been used to analyze over 100,000 zircons from the Jack Hills quartzite. Of these, ca. 7%, have an age greater than 3.8 Ga, the oldest grain being 4372 +/- 6 Ma (2[sigma]), and this age is part of a group of analyses around 4350 Ma which we interpret as the age when continental crust first began to coalesce in this region. In multi-collector mode, the analytical time taken for a single mount with 400 zircons is approximately 6 h; whereas in single-collector mode, the analytical time is ca. 17 h. With this productivity, we can produce significant numbers of zircons for statistically limited studies including correlations between age and morphology, mineral-inclusion paragenesis, as well as isotopic studies including Hf and O isotopic compositions, Pu-Xe, and Sm-Nd isotopes.

U-Pb geochronology of zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA: An integrated SEM, EMPA, TIMS, and SHRIMP study

USGS Publications Warehouse

Aleinikoff, J.N.; Wintsch, R.P.; Fanning, C.M.; Dorais, M.J.

2002-01-01

U-Pb ages for zircon and titanite from a granodioritic gneiss in the Glastonbury Complex, Connecticut, have been determined using both isotope dilution thermal ionization mass spectrometry (TIMS) and the sensitive high resolution ion microprobe (SHRIMP). Zircons occur in three morphologic populations: (1) equant to stubby, multifaceted, colorless, (2) prismatic, dark brown, with numerous cracks, and (3) elongate, prismatic, light tan to colorless. Cathodoluminescence (CL) imaging of the three populations shows simple concentric oscillatory zoning. The zircon TIMS age [weighted average of 207Pb/206Pb ages from Group 3 grains-450.5 ?? 1.6 Ma (MSWD=1.11)] and SHRIMP age [composite of 206Pb/238 U age data from all three groups-448.2 ?? 2.7 Ma (MSWD = 1.3)], are interpreted to suggest a relatively simple crystallization history. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter (BSE) images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths. Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis (EMPA) clearly distinguishes the two populations. Brown grains contain relatively high concentrations of Fe2O3, Ce2O3 (up to ~ 1.5 wt.%), Nb2O5, and Zr. Cerium concentration is positively correlated with total REE + Y concentration, which together can exceed 3.5 wt.%. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In contrast, colorless titanite (both as discrete grains and overgrowths on brown titanite) contains lower concentrations of Y, REE, Fe2O3, and Zr, but somewhat higher Al2O3 and Nb2O5. Uranium concentrations and Th/U discriminate between brown grains (typically 200-400 ppm U; all analyses but one have Th/U between about 0.8 and 2) and colorless grains (10-60 ppm U; Th/U of 0-0.17). In contrast to the zircon U-Pb age results, SHRIMP U-Pb data from titanite indicate multiple growth episodes. In brown grains, oscillatory zoned cores formed at 443 ?? 6 Ma, whereas white (in BSE) cross-cutting zones are 425 ?? 9 Ma. Colorless grains and overgrowths on brown grains yield an age of 265 ?? 8 Ma (using the Total Pb method) or 265 ?? 5 Ma (using the weighted average of the 206Pb/238U ages). However, EMPA chemical data identify zoning that suggests that this colorless titanite may preserve three growth events. Oscillatory zoned portions of brown titanite grains are igneous in origin; white cross-cutting zones probably formed during a previously unrecognized event that caused partial dissolution of earlier titanite and reprecipitation of a slightly younger generation of brown titanite. Colorless titanite replaced and grew over the magmatic titanite during the Permian Alleghanian orogeny. These isotopic data indicate that titanite, like zircon, can contain multiple age components. Coupling SHRIMP microanalysis with EMPA and SEM results on dated zones as presented in this study is an efficient and effective technique to extract additional chronologic ?? 2002 Elsevier Science B.V. All rights reserved.

Zircon U-Pb dating, Hf analysis from the Horoman perdiotite -age constraint for lithospheric process, and tectonic juxtaposition of collision root zone-

NASA Astrophysics Data System (ADS)

Okamoto, K.; Yi, K.; Wang, K. L.; Chung, S. L.

2017-12-01

Hidaka metamorphic belt, Hokkaido, Japan is known as youngest arc-arc collision in the world. It ncludes the youngest granulite and the Horoman peridotite complex in the highest grade zone. Age of these rocks have been determined by various methods (K-Ar, U-Pb, Rb-Sr). However, the age of Horoman peridotite complex has not been determined yet. Only Yoshikawa et al 1993) reported the cooling age of the complex as 23 Ma according to whole rock Rb-Sr isochron. This study has performed U-Pb dating of zircons from the Horoman peridotite, and from the paragneiss surrounding the peridotite complex in order to determine the intrusive age of the Horoman peridotite complex into the lower crustal conditions. Several zircon grains were separated from the peridotite. All zircons are homogeneous exhibiting different age group; 267-278 Ma, 33-40 Ma and 18-20 Ma. Hf isotope analysis indicates that the 267-278 Ma is juvenile age and other two are recycled. As a result of this measurement, rims of the zircons from the gneisses show that 238U-206Pb ages are 20 Ma and detrital cores are ranging from 580-510 Ma, 60-50 Ma, 46-40 Ma and 27 Ma. The rim ages are from the gneiss suffered amphibolite facies and granulite faices, and there is a consistancy with zircon rim ages (19 Ma) from the granulite (Kemp et al 2007, Usuki et al 2006 and so on). That is, granulite faices metamorphism was coeval to regional metamorphism in the lower crust at 20 Ma. The zircon ages from the peridotite was probably related to local hydration related to precipitation of phlogopite at 20 Ma, I type magma infiltration at 40 Ma and lithosphere formation at 270 Ma. It is considered that the Horoman peridotite complex was part of the lithosphere at 270 Ma, and the joined as subarc mantle prior to I type magma activity at 40 Ma, aud suffered local hydration and regional metamorphism at 20 Ma. Ref. Kemp, A.I.S., et al., 2007, Geology, 35, 807-810; Usuki, T. et al, 2006, Island Arc, 14, 503-516.

U-Pb detrital zircon dates and provenance data from the Beaufort Group (Karoo Supergroup) reflect sedimentary recycling and air-fall tuff deposition in the Permo-Triassic Karoo foreland basin

NASA Astrophysics Data System (ADS)

Viglietti, Pia A.; Frei, Dirk; Rubidge, Bruce S.; Smith, Roger M. H.

2018-07-01

Detrital zircon U-Pb age dating was used for provenance determination and maximum age of deposition for the Upper Permian (upper Teekloof and Balfour formations) and Lower Triassic (Katberg Formation) lithostratigraphic subdivisions of the Beaufort Group of South Africa's Karoo Basin. Ten samples were analysed using laser ablation - single collector - magnetic sectorfield - inductively coupled plasma - mass spectrometry (LA-SF-ICP-MS). The results reveal a dominant Late Carboniferous-Late Permian population (250 ± 5 Ma - 339 ± 5 Ma), a secondary Cambrian-Neoproterozoic (489 ± 5 Ma to 878 ± 24 Ma) population, a minor Mesoproterozoic (908 ± 24 Ma to 1308 ± 23) population, and minor occurrences of Devonian, Ordovician, Proterozoic and Archean zircon grains. Multiple lines of evidence (e.g. roundness and fragmentary nature of zircons, palaeo-current directions, and previous work), suggest the older zircon populations are related to sedimentary recycling in the Gondwanide Orogeny. The youngest and dominant population contain elongate euhedral grains interpreted to be directly derived from their protolith. Since zircons form in felsic igneous rocks, and no igneous rocks of Late Permian age occur in the Karoo Basin, these findings suggest significant input of volcanic material by ash falls. These results support sedimentological and palaeontological data for a Lopingian (Late Permian) age for the upper Beaufort Group, but contradict previous workers who retrieved Early Triassic dates from zircons in ashes for the Beaufort and Ecca Groups. Pb-loss not revealed by resolvable discordance on the concordia diagram, and metamictization of natural zircons are not factored into the conclusions of earlier workers.

The mineralogical and chronological evidences of subducted continent material in deep mantle: diamond, zircon and rutile separated from the Horoman peridotite of Japan

NASA Astrophysics Data System (ADS)

Li, Y.; Yang, J.; Nida, K.; Yamamoto, S.; Lin, Y.; Li, Q.; Tian, M.; Kon, Y.; Komiya, T.; Maruyama, S.

2017-12-01

The Horoman peridotite complex is an Alpine-type orogenic lherzolite massif of upper-mantle in the Hidaka metamorphic belt, Hokkaido, Japan. The peridotite complex is composed of dunite, harzburgite, spinel lherzolite and plagioclase lherzolite, exhibits a conspicuous layered structure, which is a product of a Cretaceous to early Paleogene arc-trench system formed by westward subduction of an oceanic plate between the paleo-Eurasian and paleo-North American Plates. Various combinations of diamond, corundum, moissanite, zircon, monazite, rutile, and kyanite have been separated from spinel harzburgite (700 kg) and lherzolite (500 kg), respectively. The carbon isotopes analyses of diamond grains by Nano-SIMS yielded significant light carbon isotopes feature as δ13 CPDB values ranging from -29.2 ‰ to -17.2 ‰, with an average of -22.8±0.32 ‰. Zircon grains occur as sub-angular to round in morphological characteristics, similar to zircons of crustal sedimentary rocks. Many zircons contain small inclusions, comprise of quartz, apatite, rutile and muscovite. The U-Pb age of zircon grains analyzed using LA-ICP-MS and SIMS gave a wide age range, from the Jurassic to Archean (ca 159 - 3131 Ma). In the zircon age histogram, four age groups were identified; the age peaks are 2385 Ma, 1890 Ma, 1618 Ma and 1212 Ma, respectively. On the other hand, U-Pb ages of rutile grains analyzed using SIMS gave a peak of 370 Ma in age histogram. The mineralogical and chronological evidences of numerous crustal minerals in peridotite of Horoman suggest that the ancient continent material was subducted in deep mantle and recycled through the upper mantle by multicycle subduction processes.

Investigating sources of ignimbrites in the Altiplano-Puna Volcanic Complex using U-Pb dating of zircons

NASA Astrophysics Data System (ADS)

Kern, J. M.; de Silva, S. L.; Schmitt, A. K.

2011-12-01

Large silicic volcanic fields (LSVFs) are thought to represent the surface expression of upper crustal batholith emplacement, with the spatiotemporal distribution of the vents and eruptions representing the development of the system. The Altiplano-Puna Volcanic Complex (APVC) in the Central Andes is a LSVF active from 11-1 Ma that erupted over 13,000 km3 of magma from large, multicyclic caldera centers and smaller ignimbrite shields during 3 distinct pulses of volcanism at 8.4, 5.5, and 4.0 Ma. Links to the magmatic system beneath are being pursued through U-Pb zircon dating of APVC ignimbrites. Initial results comprise 61 238U/206Pb zircon ages of mostly marginal crystal domains from five APVC ignimbrites-the 0.98 ± 0.03 Ma Purico, 3.96 ± 0.08 Ma Atana, 4.0 ± 0.9 Ma Toconao, 4.09 ± 0.02 Ma Puripicar, and 8.33 ± 0.06 Ma Sifon ignimbrites-dated by high-resolution secondary ionization mass spectrometry (SIMS). Each zircon analyzed was less than 350 μm in length and cathodoluminescence images reveal zonations within individual zircons, though significant core-rim age differences are rare. The ~1 Ma Purico ignimbrite displays multiple zircon age populations significantly predating the 40Ar/39Ar eruption age, but younger than ages from the nearby large-volume Atana ignimbrite erupted from La Pacana caldera. Some peaks do, however, coincide with later resurgent activity within La Pacana as expressed by the 2.7 Ma Cerro Bola dome. Zircon ages in the Atana ignimbrite are indistinguishable from its eruption, while those from the 4.0 Ma Toconao ignimbrite-the volatile-rich cap of the Atana magma chamber-contains three populations of xenocrystic zircons from the Proterozoic-Ordivician, ~13 Ma, and ~9 Ma. The ~9 Ma zircons correlate with K-Ar ages from an underlying ignimbrite, whereas the 13 Ma xenocrysts likely have a plutonic source. The Purico ignimbrite thus provides direct evidence of zircon inheritance from previous eruption cycles, while the Toconao records a much more complex history of inheritance and assimilation absent from its consanguineous counterpart, the Atana. The 4.09 Ma Puripicar and 8.33 Ma Sifon ignimbrites display a single zircon age peak up to ~ 0.5 Ma prior to the eruption age. While these data may suggest relatively long magma residence times, the time periods are significantly shorter than the repose period between episodes of ignimbrite eruptions. New U-Pb zircon ages further the understanding of the connections between LSFV ignimbrites and their underlying batholiths.

The Comparison of Detrital Zircon Ages to Point Count Provenance Analysis for the Pottsville Sandstone in the Northern Appalachian Foreland Basin Venango County, Pennsylvania

NASA Astrophysics Data System (ADS)

Loveday, S.; Harris, D. B.; Schiappa, T.; Pecha, M.

2017-12-01

The specific sources of sediments deposited in the Appalachian basin prior to and immediately following the Alleghenian orogeny has long been a topic of debate. Recent advances in U-Pb dating of detrital zircons have greatly helped to determine some of the sources of these sediments. For this study, sandstone samples were collected from the Pottsville Formation in the northern Appalachian Foreland Basin, Venango County, Pennsylvania to provide supplementary data for previous work that sought to describe the provenance of the same sediments by point counts of thin sections of the same units. Results of this previous work established that the provenance for these units was transitional recycled orogenic, including multiple recycled sediments, and that a cratonic contribution was not able to be determined clearly. The previous results suggested that the paleoenvironment was a fluvial dominated delta prograding in the northern direction. However, no geochronologic data was found during this study to confirm this interpretation. We sought to verify these results by U-Pb analysis of detrital zircons. Samples were collected from the areas where the previous research took place. U-Pb ages were found from sample at the highest elevation and lowest elevation. In the first sample, sample 17SL01 (younger sample stratigraphically), the zircons yield U-Pb age range peaks at 442-468 ma and 1037-1081 ma. The probability density plot for this specific sample displays a complete age gap from 500 ma to 811 ma. In the second sample, sample 17SL03 (older rock stratigraphically), the zircons yield U-Pb ages range peaks of 424-616 ma and 975-1057 ma. This sample doesn't show any ages younger than 424 ma and it doesn't display the sample age gap as sample 17SL01 does. The ages of zircons are consistent with thin section point counting provenance results from previous research suggesting zircon transport from the northern direction.

Large-Scale, Long-Lived Subduction of Ultrahigh-Pressure Terranes: Western Gneiss Region, Norway

NASA Astrophysics Data System (ADS)

Kylander-Clark, A. R.; Hacker, B. R.; Johnson, C. M.; Beard, B. L.; Corfu, F.; Mahlen, N. J.

2007-12-01

Recent Lu-Hf and Sm-Nd ages of garnets and a U-Pb age of zircon of eclogites from the Western Gneiss Region (WGR) ultrahigh-pressure (UHP) terrane, Norway, demonstrate that eclogite-facies metamorphism occurred over a large area (60,000 km2) for an unexpectedly long time. This observation stands in stark contrast to the general belief that continental subduction, and attendant (U)HP metamorphism, occurs over short timescales. Four HP eclogites (~700-800°C, ~2.0-2.5 GPa) from the central WGR yielded equivalent Lu-Hf ages of ~416 Ma; three of these samples gave Sm-Nd ages of ~400 Ma. Given the distribution coefficients for Lu and Sm, the older Lu-Hf ages reflect prograde growth, but are younger than the initiation of garnet crystallization. The younger Sm-Nd ages represent either eclogite-facies cooling through the blocking temperature of the Sm-Nd system or an 'average' age of garnet growth. Both cases imply >16 m.y. of eclogite- facies conditions. Two UHP eclogites (~750-850°C, ~3 GPa) from the same region yielded significantly younger, but equivalent Lu-Hf and Sm-Nd ages of ~380 Ma, which likely indicate passage through the blocking temperature of both systems up to 20 m.y. after the HP eclogites had passed through the blocking temperature. Because these eclogites are unretrogressed, their ages are the youngest known for eclogite stability in the WGR. An eclogite from the northern WGR yielded a Sm-Nd age of 413.9 ± 3.7 Ma. This could represent a different HP history than that of the central WGR: U/Pb ages in the north are also ~15 m.y. older. Two HP (~650°C, ~2 GPa) eclogites from the southern WGR yielded Lu-Hf ages of 410.2 ± 3.1 and 427.5 ± 7.7 Ma, indicating a similar garnet growth history to the central WGR eclogites. A retrogressed eclogite from the undated eastern portion of the WGR gave a 206Pb/238U age of 408.0 ± 1.7 Ma. Thermal models mirror results from similar studies (Roselle et al., 2002), and confirm that slow subduction likely produced the P-T-t record observed in the WGR. For subduction of a continent into a formerly Andean-style subduction zone, Calculated temperatures of ~700-800°C are reached at 100 km depth only at burial rates slower than 4 mm/yr. These same temperatures are achieved with slow burial (4-7 mm/yr) during intracontinental subduction.

Advanced Cyberinfrastructure for Geochronology as a Collaborative Endeavor: A Decade of Progress, A Decade of Plans

NASA Astrophysics Data System (ADS)

Bowring, J. F.; McLean, N. M.; Walker, J. D.; Gehrels, G. E.; Rubin, K. H.; Dutton, A.; Bowring, S. A.; Rioux, M. E.

2015-12-01

The Cyber Infrastructure Research and Development Lab for the Earth Sciences (CIRDLES.org) has worked collaboratively for the last decade with geochronologists from EARTHTIME and EarthChem to build cyberinfrastructure geared to ensuring transparency and reproducibility in geoscience workflows and is engaged in refining and extending that work to serve additional geochronology domains during the next decade. ET_Redux (formerly U-Pb_Redux) is a free open-source software system that provides end-to-end support for the analysis of U-Pb geochronological data. The system reduces raw mass spectrometer (TIMS and LA-ICPMS) data to U-Pb dates, allows users to interpret ages from these data, and then facilitates the seamless federation of the results from one or more labs into a community web-accessible database using standard and open techniques. This EarthChem database - GeoChron.org - depends on keyed references to the System for Earth Sample Registration (SESAR) database that stores metadata about registered samples. These keys are each a unique International Geo Sample Number (IGSN) assigned to a sample and to its derivatives. ET_Redux provides for interaction with this archive, allowing analysts to store, maintain, retrieve, and share their data and analytical results electronically with whomever they choose. This initiative has created an open standard for the data elements of a complete reduction and analysis of U-Pb data, and is currently working to complete the same for U-series geochronology. We have demonstrated the utility of interdisciplinary collaboration between computer scientists and geoscientists in achieving a working and useful system that provides transparency and supports reproducibility, allowing geochemists to focus on their specialties. The software engineering community also benefits by acquiring research opportunities to improve development process methodologies used in the design, implementation, and sustainability of domain-specific software.

U-Pb Dating of Calcite to Constrain Basinal Brine Flux Events: An Example from the Upper Midwest USA

NASA Astrophysics Data System (ADS)

Rasbury, T.; Luczaj, J.

2017-12-01

Calcite forms in a variety of settings and can be the product of surface to deep basinal fluids. As such, this mineral can uniquely record details of the fluids responsible for its formation. The forms of calcium carbonates and their stratigraphic relationships from the thin section to the regional scale give important insights on pulses of fluids. A fundamental question is the age of such fluid pulses. While calcite excludes uranium (U) from its crystal structure, some is incorporated and depending on the U/Pb ratio, this provides an opportunity for radiometric dating. Calcite crystals of various sizes and crystal habits are found in Paleozoic carbonate rocks throughout the region from the western Michigan basin to the upper Mississippi valley. These are typically associated with Mississippi Valley-type (MVT) mineralization, including galena, sphalerite, and iron sulfides, but typically post-date the main MVT event. We have analyzed a variety of these calcites and find multiple generations of calcite, separated by tens of millions of years. The initial Pb isotope ratios are similar to the isotope ratios of nearby galena, strongly suggesting a genetic relationship. Our oldest ages are 200 Ma, and we find ages ranging into the Cenozoic. Based on the Paleozoic-hosted galena Pb-isotope isoscapes from the region, the fluids may have been sourced from both the Michigan and Illinois basins. An important and unanswered question is what would cause significant fluid movement out of the basins substantially after Appalachian orogenesis. Noble gas data from brines in the Michigan Basin have a mantle component and have been suggested to be responsible for recognized elevated temperatures across the basin (Ma et al., 2009). Multiple thermal events during the Paleozoic and Mesozoic eras may have an internal heat source related to reactivation of faults of the Keweenawan Rift system below the Michigan Basin. Perhaps a mantle heat source from below episodically fluxes into the basins and displaces brines with important ore metals out of the basin. More work on the calcites will help to establish the geographic extent of the various fluid pulses in the region, which will lead to an improved understanding of the sources and paths of these fluids.

Zircon U-Pb geochronology links the end-Triassic extinction with the Central Atlantic Magmatic Province.

PubMed

Blackburn, Terrence J; Olsen, Paul E; Bowring, Samuel A; McLean, Noah M; Kent, Dennis V; Puffer, John; McHone, Greg; Rasbury, E Troy; Et-Touhami, Mohammed

2013-05-24

The end-Triassic extinction is characterized by major losses in both terrestrial and marine diversity, setting the stage for dinosaurs to dominate Earth for the next 136 million years. Despite the approximate coincidence between this extinction and flood basalt volcanism, existing geochronologic dates have insufficient resolution to confirm eruptive rates required to induce major climate perturbations. Here, we present new zircon uranium-lead (U-Pb) geochronologic constraints on the age and duration of flood basalt volcanism within the Central Atlantic Magmatic Province. This chronology demonstrates synchroneity between the earliest volcanism and extinction, tests and corroborates the existing astrochronologic time scale, and shows that the release of magma and associated atmospheric flux occurred in four pulses over about 600,000 years, indicating expansive volcanism even as the biologic recovery was under way.

CONSTRAINTS ON EXHUMATION AND SEDIMENTS PROVENANCE DURING PALEOGENE IN THE NORTHERN PYRENEES (FRANCE) USING DETRITAL AFT, ZHe AND Z(U/Pb) THERMOCHRONOLOGY

NASA Astrophysics Data System (ADS)

Filleaudeau, P.; Mouthereau, F.; Fellin, M.; Pik, R.; Lacombe, O.

2009-12-01

The Pyrenees are a doubly vergent orogenic wedge built by the convergence between the subducting Iberian microplate and the European plate lasting from late Cretaceous to early Miocene. The backbone of the Pyrenean belt (Axial Zone) consists in a stack of thrusts units composed of Paleozoic series intruded by late-Variscan granitoids. Both pro- and retro-wedge sides of the Pyrenees are fold-and-thrust belts made of Meso-Cenozoic sediments thrusted onto the Ebro and Aquitaine foreland basins. The deep structure, highlighted by the ECORS profile, shows a strong asymmetry caused by the southward migration of deformation associated with the development of a Paleogene antiformal stack emplaced during wedge growth in the Iberian plate. The present study focuses on the synorogenic deposits of the retro-foreland basin in the northern part of the belt. To examine the source rocks and quantify the exhumation rates, we combine fission track thermochronometry on detrital apatites with Helium diffusion and U/Pb thermochronometry on zircons. Due to the very high closure temperature of the U/Pb system and the wide range of age distribution, the U/Pb method, that provides zircon crystallisation ages, is a powerful tool to distinguish the various eroded sources feeding the North Pyrenean basin. Thus, we can separate grains coming from Variscan intrusive basement with ages around 310 Ma from younger grains coming from Permian or Triassic to lower Jurassic volcanics. Zircon ages of 220 Ma found in the Paleocene sandstones point to the Triassic volcanic rocks (the so-called “ophites”) as the main source of detrital grains. We infer that Paleozoic units of the Axial Zone were not outcropping in the Paleocene catchments. Exhumation rates are estimated through apatite fission track grain-age distributions and (U-Th)/He dating for two Lutetian and Bartonian synorogenic sandstone samples of the North Pyenean foreland basin. The first results obtained with AFT dating show two main grain populations, with ages ranging from Albian (around 100 Ma) to Paleocene-Eocene (50-60 Ma). These cooling ages are interpreted as related to the Albian post-rift exhumation and the syn-collisional exhumation, respectively. We finally estimate constant exhumation rates of 0.3-0.4km/Ma of the Paleozoic granitoides of the Axial Zone during the Lutetian. Since Paleocene Z(U/Pb) in the sandstones are essentially younger than in situ basement ages, we conclude that the Paleozoic basement of the Axial Zone was not exhumed at the surface before the Lutetian. This brings us new constraints on the timing of Pyrenean wedge growth along ECORS profile.

Magma evolution as seen through zircon geochemistry: an example from the Southern Adamello Batholith, N. Italy

NASA Astrophysics Data System (ADS)

Broderick, C.; Schaltegger, U.; Gerdes, A.; Frick, D.; Guenther, D.; Brack, P.

2012-04-01

Zircon is an ubiquitous accessory mineral often used for U-Pb geochronology but is also an important recorder of geochemical information. The trace element and isotopic characteristics of zircon yield potential for tracking changes in an evolving magma through time. With recent advances in U-Pb zircon geochronology, 10-100 ka to Ma timescales are observed for incremental pluton construction (Michel et al., 2008, Schaltegger et al., 2009). In observed 100 ka timescales of zircon crystallization, can zircon record the processes that produce trace element variations in a magma? This study focuses on the Val Fredda Complex (VFC) in the southern tip of the 43 to 33 Ma Adamello batholith, N. Italy. The VFC displays complex relationships among mafic melts that were injected into solidifying felsic magmas. Single zircon crystals were dated using CA-ID-TIMS. With permil uncertainties on 206Pb/238U zircon dates, zircons reveal complexities within single populations. The mafic units crystallized potential autocrystic zircons over a duration of 100 - 150ka, whereas the felsic units record up to 200ka of zircon crystallization. In order to understand these complex zircon populations, we analyzed Hf isotopes and trace elements, on the same volume of zircon used for U-Pb dating, following the TIMS-TEA method (Schoene et al., 2010). This detailed zircon study will allow us to look at how magmas are evolving with time. Hf isotopes of VFC mafic zircons reveal distinct ɛHf values between the three mafic units and their ɛHf values remain consistent through time, whereas the VFC felsic units record more complexity in their ɛHf values. We observe changes such as increasing and slight decreases in ɛHf with time which suggest different processes are occurring to produce the different felsic units. Trace element ratios in zircon reveal differences which allow us to make distinctions between felsic and mafic units (e.g. Th/U, (Lu/Gd)N, REEs). The VFC records 200 ka of zircon crystallization and our data suggests that zircons do in fact reflect changes in isotopic and in trace element signatures on 100 ka timescales. Although we observe changes in our trace elements, the TIMS-TEA method provides an average of trace element concentrations from a zircon volume, dominated by more marginal growth zones. Therefore we will compare our data with in situ methods to determine how our trace element data compares with trace elements across zircon profiles. We acknowledge funding of FNS in the frame of ProDoc Adamello 4-D. Michel et al., 2008, Geol. 36 : 459-462 ; Schaltegger et al., 2009, Earth Planet. Sci.Lett. 286: 208-218; Schoene et al., 2010, Geochim. Cosmochim. Acta 74, 7144-7159.

A new insight into Pan-African tectonics in the East-West Gondwana collision zone by U-Pb zircon dating of granites from central Madagascar

NASA Astrophysics Data System (ADS)

Nédélec, A.; Paquette, J.-L.

1998-02-01

The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U-Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as ``stratoid'' granites. These are of both mantle and crustal derivation. Their U-Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2-2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike-slip tectonic regime at ~570-560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

A new insight into Pan-African tectonics in the East-West Gondwana collision zone by U-Pb zircon dating of granites from central Madagascar

NASA Astrophysics Data System (ADS)

Paquette, Jean-Louis; Nédélec, Anne

1998-02-01

The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U-Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as "stratoid" granites. These are of both mantle and crustal derivation. Their U-Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2-2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike-slip tectonic regime at ˜570-560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

Source, evolution and emplacement of Permian Tarim Basalts: Evidence from U-Pb dating, Sr-Nd-Pb-Hf isotope systematics and whole rock geochemistry of basalts from the Keping area, Xinjiang Uygur Autonomous region, northwest China

NASA Astrophysics Data System (ADS)

Zhang, Dayu; Zhou, Taofa; Yuan, Feng; Jowitt, Simon M.; Fan, Yu; Liu, Shuai

2012-04-01

Permian basalts distribute at least 250,000 km2, and underlie the southwest Tarim Basin in Xinjiang Uygur Autonomous region, northwest China. This vast accumulation of basalt is the main part of the Tarim Large Igneous Province (LIP). The basaltic units in the Lower Permian Kupukuziman and Kaipaizileike Formations in the Keping area, Tarim Basin; were the best exposure of the Permian basalt sequence in the basin. LA-ICP-MS U-Pb dating of zircon from the basal basaltic unit in the section gives an age of 291.9 ± 2.2 Ma (MSWD = 0.30, n = 17); this age, combined with previously published geochronological data, indicates that the basalts in the Tarim Basin were emplaced between 292 Ma and 272 Ma, with about 90% of the basalts being emplaced between 292 and 287 Ma. Basalts from the Keping area have high FeOT (10.8-18.6 wt.%), low Mg#s (0.26-0.60), and exhibit primitive mantle normalized patterns with positive Pb, P and Ti but negative Zr, Y and Ta anomalies. The basalts from both formations have similar 206Pb/204Pb (18.192-18.934), 207Pb/204Pb (15.555-15.598) and 208Pb/204Pb (38.643-38.793) ratios. The basalts also have high ɛSr(t) (45.7-62.1), low ɛNd(t) (-3.6 to -2.2) and low zircon ɛHf(t) (-4.84 to -0.65) values. These characteristics are typical of alkali basalts and suggest that the basalts within the Tarim Basin were derived from an OIB-type mantle source and interacted with enriched mantle (EMI-type) before emplacement. Rare earth element systematics indicate that the parental melts for the basalts were high-degree partial melts derived from garnet lherzolite mantle at the base of the lithosphere. Prior to emplacement, the Tarim Permian Basalts (TPB) underwent fractional crystallization and assimilated crustal material; the basalts were finally emplaced during crustal extension in an intra-plate setting. The wide distribution, deep source and high degree partial melting of the TPB was consistent with a mantle plume origin. The TPB and other coeval igneous rocks in the Tarim Basin constitute a Permian LIP formed by a mantle plume in a similar fashion to the plume-related Emeishan LIP in southwest China.

U-Pb, Re-Os and Ar-Ar dating of the Linghou polymetallic deposit, Southeastern China: Implications for metallogenesis of the Qingzhou-Hangzhou metallogenic belt

NASA Astrophysics Data System (ADS)

Tang, Yanwen; Xie, Yuling; Liu, Liang; Lan, Tingguan; Yang, Jianling; Sebastien, Meffre; Yin, Rongchao; Liang, Songsong; Zhou, Limin

2017-04-01

The Qingzhou-Hangzhou metallogenic belt (QHMB) in Southeastern China has gained increasingly attention in recent years. However, due to the lack of reliable ages on intrusions and associated deposits in this belt, the tectonic setting and metallogenesis of the QHMB have not been well understood. The Linghou polymetallic deposit in northwestern Zhejiang Province is one of the typical deposits of the QHMB. According to the field relationships, this deposit consists of the early Cu-Au-Ag and the late Pb-Zn-Cu mineralization stages. Molybdenite samples with a mineral assemblage of molybdenite-chalcopyrite-pyrite ± quartz are collected from the copper mining tunnel near the Cu-Au-Ag ore bodies. Six molybdenite samples give the Re-Os model ages varying from 160.3 to 164.1 Ma and yield a mean age of 162.2 ± 1.4 Ma for the Cu-Au-Ag mineralization. Hydrothermal muscovite gives a well-defined Ar-Ar isochron age of 160.2 ± 1.1 Ma for the Pb-Zn-Cu mineralization. Three phases of granodioritic porphyry have been distinguished in this deposit, and LA-ICP-MS zircon U-Pb dating shows that they have formed at 158.8 ± 2.4 Ma, 158.3 ± 1.9 Ma and 160.6 ± 2.1 Ma, comparable to the obtained ages of the Cu-Au-Ag and Pb-Zn-Cu mineralization. Therefore, these intrusive rocks have a close temporal and spatial relationship with the Cu-Au-Ag and Pb-Zn-Cu ore bodies. The presences of skarn minerals (e.g., garnet) and vein-type ores, together with the previous fluid inclusion and H-O-C-S-Pb isotopic data, clearly indicate that the Cu-Au-Ag and Pb-Zn-Cu mineralization are genetically related to these granodiorite porphyries. This conclusion excludes the possibility that this deposit is of ;SEDEX; type and formed in a sag basin of continental rifts setting as previously proposed. Instead, it is proposed that the Linghou polymetallic and other similar deposits in the QHMB, such as the 150-160 Ma Yongping porphyry-skarn Cu-Mo, Dongxiang porphyry? Cu, Shuikoushan/Kangjiawang skarn Pb-Zn, Fozichong skarn Pb-Zn and Dabaoshan porphyry-skarn deposits are of magmatic-hydrothermal origin and likely formed in a subduction-related setting. This work provides new insight that these intrusion-related deposits (e.g., porphyry and skarn types) of middle to late Jurassic age can be the most important targets for exploration in the QHMB.

Controls on Cenozoic exhumation of the Tethyan Himalaya from fission-track thermochronology and detrital zircon U-Pb geochronology in the Gyirong basin area, southern Tibet

NASA Astrophysics Data System (ADS)

Shen, Tianyi; Wang, Guocan; Leloup, Philippe Hervé; van der Beek, Peter; Bernet, Matthias; Cao, Kai; Wang, An; Liu, Chao; Zhang, Kexin

2016-07-01

The Gyirong basin, southern Tibet, contains the record of Miocene-Pliocene exhumation, drainage development, and sedimentation along the northern flank of the Himalaya. The tectonic controls on basin formation and their potential link to the South Tibetan Detachment System (STDS) are not well understood. We use detrital zircon (ZFT) and apatite (AFT) fission-track analysis, together with detrital zircon U-Pb dating to decipher the provenance of Gyirong basin sediments and the exhumation history of the source areas. Results are presented for nine detrital samples of Gyirong basin sediments (AFT, ZFT, and U-Pb), two modern river-sediment samples (ZFT and AFT), and six bedrock samples (ZFT) from transect across the Gyirong fault bounding the basin to the east. The combination of detrital zircon U-Pb and fission-track data demonstrates that the Gyirong basin sediments were sourced locally from the Tethyan Sedimentary Sequence. This provenance pattern indicates that deposition was controlled by the Gyirong fault, active since 10 Ma, whose vertical throw was probably < 5000 m, rather than being controlled by normal faults associated with the STDS. The detrital thermochronology data contain two prominent age groups at 37-41 and 15-18 Ma, suggesting rapid exhumation at these times. A 15-18 Ma phase of rapid exhumation has been recorded widely in both southern Tibet and the Himalaya. A possible interpretation for such a major regional exhumation event might be detachment of the subducting Indian plate slab during the middle Miocene, inducing dynamic uplift of the Indian plate overriding its own slab.

Metallogeny of precious and base metal mineralization in the Murchison Greenstone Belt, South Africa: indications from U-Pb and Pb-Pb geochronology

NASA Astrophysics Data System (ADS)

Jaguin, J.; Poujol, M.; Boulvais, P.; Robb, L. J.; Paquette, J. L.

2012-10-01

The 3.09 to 2.97 Ga Murchison Greenstone Belt is an important metallotect in the northern Kaapvaal Craton (South Africa), hosting several precious and base metal deposits. Central to the metallotect is the Antimony Line, striking ENE for over 35 km, which hosts a series of structurally controlled Sb-Au deposits. To the north of the Antimony Line, hosted within felsic volcanic rocks, is the Copper-Zinc Line where a series of small, ca. 2.97 Ga Cu-Zn volcanogenic massive sulfide (VMS)-type deposits occur. New data are provided for the Malati Pump gold mine, located at the eastern end of the Antimony Line. Crystallizations of a granodiorite in the Malati Pump Mine and of the Baderoukwe granodiorite are dated at 2,964 ± 7 and 2,970 ± 7 Ma, respectively (zircon U-Pb), while pyrite associated with gold mineralization yielded a Pb-Pb age of 2,967 ± 48 Ma. Therefore, granodiorite emplacement, sulfide mineral deposition and gold mineralization all happened at ca. 2.97 Ga. It is, thus, suggested that the major styles of orogenic Au-Sb and the Cu-Zn VMS mineralization in the Murchison Greenstone Belt are contemporaneous and that the formation of meso- to epithermal Au-Sb mineralization at fairly shallow levels was accompanied by submarine extrusion of felsic volcanic rocks to form associated Cu-Zn VMS mineralization.

Les minéralisations Cu_(Ni_Bi_U_Au_Ag) d'Ifri (district du Haut Seksaoua, Maroc) : apport de l'étude texturale au débat syngenèse versus épigenèseThe Cu_(Ni_Bi_U_Au_Ag) mineralization of Ifri ('Haut Seksaoua' district, Morocco): contribution of a textural study to the discussion syngenetic versus epigenetic

NASA Astrophysics Data System (ADS)

Barbanson, Luc; Chauvet, Alain; Gaouzi, Aziz; Badra, Lakhifi; Mechiche, Mohamed; Touray, Jean Claude; Oukarou, Saı̈d

2003-11-01

The Cu ore of Ifri is a chalcopyrite stockwork hosted by Cambrian formations and was until now interpreted as a syngenetic massive sulphide deposit. Textural studies highlight two generations of pyrite early (Py I) and late (Py II) with respect to the regional deformation. The chalcopyrite stockwork overprinted Py II, outlining the epigenetic nature of the Cu mineralization. Regarding the origin of Cu-depositing fluids, the presence in the stockwork paragenesis of an U, W, Sn association and preliminary Pb/Pb dating of a brannerite belonging to this association suggest a contribution of the Tichka granite. To cite this article: L. Barbanson et al., C. R. Geoscience 335 (2003).

U-Pb dating and emplacement history of granitoid plutons in the northern Sanandaj-Sirjan Zone, Iran

NASA Astrophysics Data System (ADS)

Mahmoudi, Shahryar; Corfu, Fernando; Masoudi, Fariborz; Mehrabi, Behzad; Mohajjel, Mohammad

2011-05-01

The Sanandaj-Sirjan Zone (SSZ), which runs parallel to the Zagros fold and thrust belt of Iran, underwent a multistage evolution starting with Neotethys initiation, its subsequent subduction below the Iranian continental crust, and eventual closure during convergence of Arabia towards central Iran. Plutonic complexes are well developed in the northern part of the SSZ and we have dated a number of them by ID-TIMS U-Pb on zircon. The new data record the following events: a Mid Jurassic period that formed the Boroujerd Plutonic Complex (169 Ma), the Astaneh Pluton (168 Ma) and the Alvand Pluton (165 Ma); Late Jurassic emplacement of the Gorveh Pluton (157-149 Ma); Mid Cretaceous (109 Ma) formation of a I-type phase in the Hasan Salary Pluton near Saqqez, followed by Early Paleocene (60 Ma) intrusion of A-type granite in the same pluton; and the youngest intrusive event recorded so far in the SSZ with the intrusion of granite in the Gosheh-Tavandasht Complex near Boroujerd at 34.9 Ma. These different events reflect specific stages of subduction-related magmatism prior to the eventual Miocene collision between the two continental blocks.

ESR dating of barite in sulphide deposits formed by the sea-floor hydrothermal activities.

PubMed

Toyoda, Shin; Fujiwara, Taisei; Uchida, Ai; Ishibashi, Jun-ichiro; Nakai, Shun'ichi; Takamasa, Asako

2014-06-01

Barite is a mineral newly found to be practically useful for electron spin resonance (ESR) dating of sulphide deposits formed by the sea-floor hydrothermal activities. The recent studies for the properties of the ESR dating signal in barite are summarised in the present paper as well as the formulas for corrections for accurate dose-rate estimation are developed including the dose-rate conversion factors, shape correction for gamma-ray dose and decay of (226)Ra. Although development of the techniques for ESR dating of barite has been completed, further comparative studies with other dating techniques such as U-Th and (226)Ra-(210)Pb dating are necessary for the technique to be widely used. © The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

Allanite age-dating: Non-matrix-matched standardization in quadrupole LA-ICP-MS

NASA Astrophysics Data System (ADS)

Burn, M.; Lanari, P.; Pettke, T.; Engi, M.

2014-12-01

Allanite Th-U-Pb age-dating has recently been found to be powerful in unraveling the timing of geological processes such as the metamorphic dynamics in subduction zones and crystallization velocity of magmas. However, inconsistencies among analytical techniques have raised doubts about the accuracy of allanite age data. Spot analysis techniques such as LA-ICP-MS are claimed to be crucially dependent on matrix-matched standards, the quality of which is variable. We present a new approach in LA-ICP-MS data reduction that allows non-matrix-matched standardization via well constrained zircon reference materials as primary standards. Our data were obtained using a GeoLas Pro 193 nm ArF excimer laser ablation system coupled to an ELAN DRC-e quadrupole ICP-MS. We use 32 μm and 24 μm spot sizes; laser operating conditions of 9 Hz repetition rate and 2.5 J/cm2 fluence have proven advantageous. Matrix dependent downhole fractionation evolution is empirically determined by analyzing 208Pb/232Th and 206Pb/238U and applied prior to standardization. The new data reduction technique was tested on three magmatic allanite reference materials (SISSb, CAPb, TARA); within error these show the same downhole fractionation evolution for all allanite types and in different analytical sessions, provided measurement conditions remain the same. Although the downhole evolution of allanite and zircon differs significantly, a link between zircon and allanite matrix is established by assuming CAPb and TARA to be fixed at the corresponding reference ages. Our weighted mean 208Pb/232Th ages are 30.06 ± 0.22 (2σ) for SISSb, 275.4 ± 1.3 (2σ) for CAPb, and 409.9 ± 1.8 (2σ) for TARA. Precision of single spot age data varies between 1.5 and 8 % (2σ), dependent on spot size and common lead concentrations. Quadrupole LA-ICP-MS allanite age-dating has thus similar uncertainties as do other spot analysis techniques. The new data reduction technique is much less dependent on quality and homogeneity of allanite standard reference materials. This method of correcting for matrix-dependent downhole fractionation evolution opens new possibilities in the field of LA-ICP-MS data acquisition, e.g. the use of a NIST standard glass to date all material types given a set of well constrained reference materials.

187Re - 187Os nuclear geochronometry: age dating with permil precision

NASA Astrophysics Data System (ADS)

Roller, Goetz

2016-04-01

Recently, 187Re - 187Os nuclear geochronometry, a new dating method combining ideas of nuclear astrophysics with geochronology, has successfully been used to calculate two-point-isochron (TPI) ages for Devonian black gas shales using the isotopic signature of an r-process geochronometer as one data point in a TPI diagram [1]. Based upon a nuclear production ratio 187Re/188Os = 5.873, TPI ages were calculated for 12 SDO-1 (Devonian Ohio Shale, Appalachian Basin) aliquants, for which repeated Re-Os measurements are reported in the literature [2]. TPI ages range from 384.5 ± 2.7 Ma (187Os/188Osi = 0.29413 ± 0.00023) to 387.7 ± 2.1 Ma (187Os/188Osi = 0.29407 ± 0.00019) with a mean of 386.67 ± 1.79 Ma). The result is consistent with the isochronous age from the 12 aliquants alone (386 ± 16 Ma, 187Os/188Osi = 0.31±0.31), which is bracketed by U-Pb ages for the Belpre Ash (381.1 ± 3.3 Ma) and the Tioga Ash bed (390.0 ± 2.5 Ma) [3] from the Appalachian Basin. Hence, SDO-1 can be assigned to the Givetian stage (varcus-zone) of the Middle Devonian, close to the Eifelian/Givetian boundary (using the time-scale of [3] or [4]). If an age is calculated from an isochron diagram for the 12 aliquants including the nuclear geochronometer, a permil precision can be achieved, an interesting feature with respect to any effort towards calibrating the Geologic Timescale. Additionally, a Th/U evolution (or: Th/U-time) diagram can be plotted using U-Pb zircon age data and Th/U ratios from volcanic rocks and ashes reported in the literature [3] for specific Devonian samples from the Appalachian Basin. Since the Re-Os age obtained for SDO-1 can also be connected to its Th/U ratio, it turns out, that Th/U ratios might be helpful age indicators, as demonstrated for the Devonian using the U-Pb and Re-Os datasets. [1] Roller (2015), GSA Abstr. with Programs 47, #248-14. [2] Du Vivier et al. (2014), Earth Planet. Sci. Lett. 389, 23 - 33. [3] Tucker et al. (1998), Earth Planet. Sci. Lett. 158, 175 - 186. [4] Kaufmann (2006), Earth-Sci. Revs. 76, 175 - 190.

A century long sedimentary record of anthropogenic lead (Pb), Pb isotopes and other trace metals in Singapore.

PubMed

Chen, Mengli; Boyle, Edward A; Switzer, Adam D; Gouramanis, Chris

2016-06-01

Reconstructing the history of metal deposition in Singapore lake sediments contributes to understanding the anthropogenic and natural metal deposition in the data-sparse Southeast Asia. To this end, we present a sedimentary record of Pb, Pb isotopes and eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U and Zn) from a well-dated sediment core collected near the depocenter of MacRitchie Reservoir in central Singapore. Before the 1900s, the sedimentary Pb concentration was less than 2 mg/kg for both soil and sediment, with a corresponding (206)Pb/(207)Pb of ∼1.20. The Pb concentration increased to 55 mg/kg in the 1990s, and correspondingly the (206)Pb/(207)Pb decreased to less than 1.14. The (206)Pb/(207)Pb in the core top sediment is concordant with the (206)Pb/(207)Pb signal of aerosols in Singapore and other Southeast Asian cities, suggesting that Pb in the reservoir sediment was mainly from atmospheric deposition. Using the Pb concentration in the topmost layer of sediment, the estimated atmospheric Pb flux in Singapore today is ∼1.6 × 10(-2) g/m(2) yr. The concentrations of eleven other metals preserved in the sediment were also determined. A principal component analysis showed that most of the metals exhibit an increasing trend towards 1990s with a local concentration peak in the mid-20(th) century. Copyright © 2016. Published by Elsevier Ltd.

Thermochronological modeling of the age of Vologda crystalline basement of the Russian platform

NASA Astrophysics Data System (ADS)

Gerasimov, V. Yu.; Petrov, D. B.; Lebedev, V. A.

2010-05-01

The results of the complex petrological and isotope-geochronological study of the crystalline rock from the deep drilling hall of the south of Vologda segment are presented in this work. The crystalline basement of the platform in Vologda region lie in a depth 2.5 km and represented by high alumina mica schist. The thick sedimentary cover consists of vendian and phanerozoic sediments. Upper level covered by quaternary glacial deposits up to 50 m. A core sample from the borehole of Fedotovo village was obtained from the depth 2600 m. It is fine-medium grained metamorphic mica schist with sillimanite. The mineral assemblage represented by association: Pl-Bt-Ms-Sil-Qtz-Mag +Zrn +Mnz. The metamorphic schist of the crystalline basement contains several radio isotope sensors. There are two rock forming potassium reach mica, - biotite (Bt) and muscovite (Ms) and accessories monazite (Mnz), - the phosphate of REE enriched by Th and U. It was a reason why traditional K-Ar isotope dating method in the combination with electron microprobe U-Th-Pb dating method CHIME [Suzuki et al. 1991] was used for Vologda metapelite rocks dating. In addition to geochronology, the detailed petrological investigation using electron microprobe allowed also to determine thermodynamic parameters of metamorphic system with a help of the mineral thermobarometry and finally estimate the age of the metamorphic thermal event using experimental diffusion data of Ar and Pb in minerals [Gerasimov et al. 2004]. The temperature of the regional metamorphism was estimated using Bt+Mag+Qtz and Bt+Ms geothermometers [Glassley 1983, Hoisch 1989]. Taking into account the field of the sillimanite P-T stability it is possible to conclude that the peak of metamorphism was reached at temperature about ТоС=550+/-30° C and pressure Р=4+/-1 kbar. Isotope thermochronology of the sample demonstrate nearly Svecofenian age 1.7-1.8 Ga of Vologda crystalline basement. K-Ar isotope dating of black and white mica demonstrates regular progression of ages in a concordance with closure temperature of each mineral. The apparent Bt age is about 1670 Ma and Ms age is 1710 Ma. The estimation of closure temperature (Tc) for each of the minerals using Dodsn's theory [Dodson 1973] and DCT computer program with concordance procedure of cooling rate simulation for the two coexisting minerals demonstrate value 340° C and и 460° C for Bt and Ms respectively. The rate of cooling in this temperature range is about 3° C/Ma and time span between closure temperatures of the two micas is about 40 Ma. The value of the cooling rate is a very typical for regional metamorphism conditions. The linear extrapolation of the simulated time-temperature trend to the thermal peak of the regional metamorphism (estimated by mineral equilibriums at 550° C) demonstrates that cooling of the metamorphic system from the thermal peak to 460° C(closure temperature of Ms) takes about 30-40 Ma. It is a value of systematic thermochronological correction to the muscovite apparent K-Ar age which has to be added to estimate the age of regional metamorphism, after that we finally receive the age 1750+/-40 Ma. U-Th-Pb system of monazite grains was tested by electron microprobe JEOL 8200 in IGEM RAS using CHIME method. The analysis of 8 grains demonstrated averaged value of age 1790+/-55 Ma. It is in a very good agreement with K-Ar isotope dating results. Moreover, the estimation of monazite closure temperature using experimental data of Pb diffusion [Smith & Giletti 1997] shows the value Tc=540-560° C which almost exactly corresponds to the peak temperature of regional metamorphism. It is also an explanation of the very close results of dating in different isotope systems (conservative U-Th-Pb system of monazite and flexible K-Ar system) in the condition of slow cooling and demonstrates the thermochronological modeling effectiveness.

U-Pb zircon geochronology of the Paleoproterozoic Tagragra de Tata inlier and its Neoproterozoic cover, western Anti-Atlas, Morocco

USGS Publications Warehouse

Walsh, G.J.; Aleinikoff, J.N.; Benziane, F.; Yazidi, A.; Armstrong, T.R.

2002-01-01

New U-Pb zircon data obtained by sensitive high resolution ion microprobe (SHRIMP) from the Tagragra de Tata inlier in the western Anti-Atlas, Morocco establish Paleoproterozoic ages for the basement schists, granites, and metadolerites, and a Neoproterozoic age for an ignimbrite of the Ouarzazate Series in the cover sequence. The age of interbedded felsic metatuff in the metasedimentary and metavolcanic sequence of the basement schists is 2072 ?? 8 Ma. This date represents: (1) the first reliable age from the metasedimentary and metavolcanic sequence; (2) the oldest reliable age for the basement of the Anti-Atlas; (3) the first date on the timing of deposition of the sediments on the northern edge of the Paleoproterozoic West African craton; (4) a lower age limit on deformation during the Eburnean orogeny; and (5) the first date obtained from the non-granitic Paleoproterozoic basement of Morocco. Ages of 2046 ?? 7 Ma (Targant granite) and 2041 ?? 6 Ma (Oudad granite) support earlier interpretations of a Paleoproterozoic Eburnean igneous event in the Anti-Atlas. The granites post-date the Eburnean D1 deformation event in the Paleoproterozoic schist sequence, and place a ???2046 Ma limit on short-lived Eburnean deformation in the area. Cross-cutting metadolerite is 2040 ?? 6 Ma; this is the first date from a metadolerite in the western Anti-Atlas. All of the dolerites in the area post-date emplacement of the two granites and the new age constrains the onset of late- or post-Eburnean extension. Ignimbrite of the Ouarzazate Series, immediately above the Paleoproterozoic basement is 565 ?? 7 Ma. This Neoproterozoic age agrees with ages of similar volcanic rocks elsewhere from the Ouarzazate Series. The date also agrees with the ages of associated hypabyssal intrusions, and marks the second and final stage of Pan-African orogenic activity in the western Anti-Atlas. ?? 2002 Elsevier Science B.V. All rights reserved.

Post-collisional alkaline magmatism as gateway for metal and sulfur enrichment of the continental lower crust

NASA Astrophysics Data System (ADS)

Fiorentini, Marco L.; LaFlamme, Crystal; Denyszyn, Steven; Mole, David; Maas, Roland; Locmelis, Marek; Caruso, Stefano; Bui, Thi-Hao

2018-02-01

Mafic and ultramafic magmas that intrude into the lower crust can preserve evidence for metal and sulfur transfer from the lithospheric mantle into the lower continental crust. Here we focus on a series of ultramafic, alkaline pipes in the Ivrea Zone (NW Italy), which exposes deeply buried (6-11 kbar), migmatitic metasedimentary rocks intruded by voluminous basaltic magmas of the Mafic Complex, a major crustal underplating event precisely dated via U/Pb CA-IDTIMS on zircon at 286.8 ± 0.4 Ma. The ultramafic pipes postdate the Mafic Complex and from 100 to 300 m wide cumulate-rich conduits. They are hydrated and carbonated, have unusually high incompatible element concentrations and contain blebby and semi-massive Ni-Cu-PGE sulfide mineralisation. The sulfides occur as coarse intergranular nodules (>10 mm) and as small intragranular blebs (<1 mm) hosted in olivine, and have homogeneous, mantle-like δ34S (+1.35 ± 0.25‰). This homogeneity suggests that the pipes reached sulfide supersaturation without addition of crustal sulfur, and that the δ34S signature is representative of the continental lithospheric mantle. One of the pipes, the 249 Ma Valmaggia pipe, carries a very distinctive Sr-Nd-Hf-Pb isotopic composition in its core (87Sr/86Sr 0.70250, εNd-18, εHf-18, 206Pb/204Pb 16.0, 207Pb/204Pb 15.16, 208Pb/204Pb 35.87), very different from the margin of this pipe and from other pipes that have higher 87Sr/86Sr, εNd and 206Pb/204Pb. The unusual isotopic composition of the Valmaggia pipe requires a source with long-term (2500-1500 million years) U-, Th- and Rb-depletion and LREE enrichment. Such compositions are found in Late Archean/Early Proterozoic granulites and lower crustal xenoliths. We suggest that the unusual isotopic composition of the Valmaggia pipe reflects contamination of the mantle source of the pipe with a crustal component that is neither represented in the local Paleozoic crust nor in the isotopically anomalous hydrated mantle inferred as the source of the large-volume mafic underplate that formed the Mafic Complex. During post-collisional gravitational collapse of the Variscan Orogen, this source produced the alkaline, metal (Ni, Cu, PGE)- and volatile (H2O, CO2, S)-rich mafic-ultramafic magma that formed the deep-crustal intrusion at Valmaggia. U/Pb dating of other chemically and geologically comparable pipes in the area shows that this process was active over at least 40 Ma. The Ivrea pipes illustrate how the lower continental crust can be fertilised with mantle-derived metals and volatiles, which are available for later remobilisation into upper-crustal ore systems. World-class mineral deposits along the margins of lithospheric blocks may thus be the result of both favourable crustal architecture (focussing of magmas and fluids) and localised volatile and metal enrichment of the lower crust related to mantle-derived hydrous metasomatism.

Internal structures and dating of non-sulphide Zn deposits using rock magnetism: insights from the Moroccan High Atlas

NASA Astrophysics Data System (ADS)

Charles, Nicolas; Choulet, Flavien; Sizaret, Stanislas; Chen, Yan; Barbanson, Luc; Ennaciri, Aomar; Badra, Lakhlifi; Branquet, Yannick

2016-01-01

The renewal of interest in Zn-Pb non-sulphide ores has been induced by mineral processing improvement and leads to new exploration and mining projects in the world. Although the mineralogy is often precisely known, and despite several studies linking ore deposition to regional tectonics, absolute dating of non-sulphide stages is rare and structure of ore bodies was largely disregarded. Geochronological data from non-sulphide ores are essential to timely constrain alteration episodes and to insert supergene ore genesis in the climate and tectonic evolution of the metallogenic province. The access to internal organization of ore could reveal post-mineralization episodes related to supergene evolution. Thus, a rock magnetism study combining anisotropy of magnetic susceptibility (AMS) and palaeomagnetism was performed on four non-sulphide deposits from the Moroccan High Atlas. AMS generally shows similar horizontal magnetic fabrics for ores and the clayey and carbonaceous internal sediments filling karstic cavities. The palaeomagnetic directions of ores and internal sediments are compatible, and the calculated poles are consistent with the last 30 Ma of the Africa apparent polar wander path, with an upper age at 0.78 Ma. The proposed three-step scenario is placed within the evolution of the Moroccan High Atlas belt. Deposition of primary sulphides is contemporaneous with opening of the Tethyan and Atlantic oceans. During the Tertiary, intracontinental deformation gave rise to the High Atlas fold-and-thrust belt and to regional uplift. Finally, Zn-Pb sulphides hosted in carbonates experienced oxidation under an arid climate to form karst-related Zn-Pb non-sulphide ores. These promising results pave the way for an efficient method to constrain the internal fabrics and age of Zn supergene deposits.

Fission track dating of kimberlitic zircons

NASA Astrophysics Data System (ADS)

Haggerty, Stephen E.; Raber, Ellen; Naeser, Charles W.

1983-04-01

The only reliable method for dating kimberlites at present is the lengthy and specialized hydrothermal procedure that extracts 206Pb and 238U from low-uranium zircons. This paper describes a second successful method by fission track dating of large single-crystal zircons, 1.0-1.5 cm in dimension. The use of large crystals overcomes the limitations imposed in conventional fission track analysis which utilizes crushed fragments. Low track densities, optical track dispersion, and the random orientation of polished surfaces in the etch and irradiation cycle are effectively overcome. Fission track ages of zircons from five African kimberlites are reported, from the Kimberley Pool (90.3 ± 6.5 m.y.), Orapa (87.4 ± 5.7 and 92.4 ± 6.1 m.y.), Nzega (51.1 ± 3.8 m.y.), Koffiefontein (90.0 ± 8.2 m.y.), and Val do Queve (133.4 ± 11.5 m.y.). In addition we report the first radiometric ages (707.9 ± 59.6 and 705.5 ± 61.0 m.y.) of crustal zircons from kimberlites in northwest Liberia. The fission track ages agree well with earlier age estimates. Most of the zircons examined in this study are zoned with respect to uranium but linear correlations are established (by regression analysis) between zones of variable uranium content, and within zones of constant uranium content (by analysis of variance). Concordance between the fission track method and the U/Pb technique is established and we concluded that track fading from thermal annealing has not taken place. Kimberlitic zircons dated in this study, therefore, record the time of eruption.

Fission track dating of kimberlitic zircons

USGS Publications Warehouse

Haggerty, S.E.; Raber, E.; Naeser, C.W.

1983-01-01

The only reliable method for dating kimberlites at present is the lengthy and specialized hydrothermal procedure that extracts 206Pb and 238U from low-uranium zircons. This paper describes a second successful method by fission track dating of large single-crystal zircons, 1.0-1.5 cm in dimension. The use of large crystals overcomes the limitations imposed in conventional fission track analysis which utilizes crushed fragments. Low track densities, optical track dispersion, and the random orientation of polished surfaces in the etch and irradiation cycle are effectively overcome. Fission track ages of zircons from five African kimberlites are reported, from the Kimberley Pool (90.3 ?? 6.5 m.y.), Orapa (87.4 ?? 5.7 and 92.4 ?? 6.1 m.y.), Nzega (51.1 ?? 3.8 m.y.), Koffiefontein (90.0 ?? 8.2 m.y.), and Val do Queve (133.4 ?? 11.5 m.y.). In addition we report the first radiometric ages (707.9 ?? 59.6 and 705.5 ?? 61.0 m.y.) of crustal zircons from kimberlites in northwest Liberia. The fission track ages agree well with earlier age estimates. Most of the zircons examined in this study are zoned with respect to uranium but linear correlations are established (by regression analysis) between zones of variable uranium content, and within zones of constant uranium content (by analysis of variance). Concordance between the fission track method and the U/Pb technique is established and we concluded that track fading from thermal annealing has not taken place. Kimberlitic zircons dated in this study, therefore, record the time of eruption. ?? 1983.

Tracking the evolution of a giant magmatic system from assembly to supereruption

NASA Astrophysics Data System (ADS)

Wotzlaw, J. F.; Schaltegger, U.; Frick, D. A.; Dungan, M. A.; Gerdes, A.; Günther, D.

2012-12-01

The chemical and physical characteristics of large magma bodies prior to eruption are closely related consequences of the balance between the rate of magma influx and the rate of cooling. The products of caldera-related silicic supereruptions, and less voluminous associated precursor and post-caldera activity preserve information about the thermal and chemical states of the system at the moment of evacuation of the magma chamber. Numerous studies of the Oligocene Fish Canyon Tuff (5000 km3; Colorado, USA) and related eruptive products of the La Garita caldera have addressed the origin and evolution of large-volume crystal-rich magmas. We use the presence of zircon in all eruptive products of the Fish Canyon magmatic system to gain a high-resolution geochronologic control on the thermal evolution of the magmatic system by using the trace element composition of U-Pb dated zircons as a proxy for magma crystallinity. New analytical protocols permit to obtain age, chemical and isotopic information from the exact same volume of single zircons (Schoene et al., 2010, GCA) allowing us to trace magma crystallinity as a function of time. Zircon U-Pb dates record ~400,000 years of crystallization. Variations in trace element composition in U-Pb dated zircons can be attributed to trace element fractionation imposed by co-crystallization of titanite. Due to the relatively high modal abundance of titanite in Fish Canyon magma and the extreme compatibility of the rare earth elements (REE) in this phase, titanite is the major control on many trace element concentrations and ratios (e.g. Yb/Dy) in coexisting zircons via varying degrees of depletion of these elements in the host melt. Modeling of compositional variations as a result of fractional crystallization suggests that the range of zircon compositions can be explained by 50-70% crystallization of a fractionating assemblage containing 0.4-0.8 vol.% titanite. The period of cooling centered around 28.4 Ma and is marked by an apparent crystallinity that is ~30% higher than at the time of eruption, implying that the Fish Canyon magma was perilously close to complete solidification, hence 'plutonic death', prior to late reheating. We estimate the duration of the reheating event from the age difference of the chemically most evolved (highest Yb/Dy) and the youngest dated zircon to be ~170 ka. This duration for reactivation of the near solidus crystal mush is in excellent agreement with estimates derived from numerical modeling of remelting by upward percolation of a hot gas-phase derived from underplated mafic magma ('gas sparging'; Bachmann and Bergantz, 2003, Geology). This investigation, which is the first in which high resolution U-Pb zircon geochronology has been coupled with geochemical modeling of systematic compositional variations in an accessory mineral, ties together many of the threads which have been developed independently during previous studies of the Fish Canyon magmatic system and places related petrogenetic processes into an absolute time frame.

Feldspar palaeo-isochrons from early Archaean TTGs: Pb-isotope evidence for a high U/Pb terrestrial Hadean crust

NASA Astrophysics Data System (ADS)

Kamber, B. S.; Whitehouse, M. J.; Moorbath, S.; Collerson, K. D.

2001-12-01

Feldspar lead-isotope data for 22 early Archaean (3.80-3.82 Ga) tonalitic gneisses from an area south of the Isua greenstone belt (IGB),West Greenland, define a steep linear trend in common Pb-isotope space with an apparent age of 4480+/-77 Ma. Feldspars from interleaved amphibolites yield a similar array corresponding to a date of 4455+/-540 Ma. These regression lines are palaeo-isochrons that formed during feldspar-whole rock Pb-isotope homogenisation a long time (1.8 Ga) after rock formation but confirm the extreme antiquity (3.81 Ga) of the gneissic protoliths [1; this study]. Unlike their whole-rock counterparts, feldspar palaeo-isochrons are immune to rotational effects caused by the vagaries of U/Pb fractionation. Hence, comparison of their intercept with mantle Pb-isotope evolution models yields meaningful information regarding the source history of the magmatic precursors. The locus of intersection between the palaeo-isochrons and terrestrial mantle Pb-isotope evolution lines shows that the gneissic precursors of these 3.81 Ga gneisses were derived from a source with a substantially higher time-integrated U/Pb ratio than the mantle. Similar requirements for a high U/Pb source have been found for IGB BIF [2], IGB carbonate [3], and particularly IGB galenas [4]. Significantly, a single high U/Pb source that separated from the MORB-source mantle at ca. 4.3 Ga with a 238U/204Pb of ca. 10.5 provides a good fit to all these observations. In contrast to many previous models based on Nd and Hf-isotope evidence we propose that this reservoir was not a mantle source but the Hadean basaltic crust which, in the absence of an operating subduction process, encased the early Earth. Differentiation of the early high U/Pb basaltic crust could have occurred in response to gravitational sinking of cold mantle material or meteorite impact, and produced zircon-bearing magmatic rocks. The subchondritic Hf-isotope ratios of ca. 3.8 Ga zircons support this model [5] provided that the redetermined 176Lu decay constant of Scherer et al. [6] is correct. Our model of a stable basaltic Hadean shell for the pre-plate tectonic era explicitly refutes operation of processes such as sediment recycling or melting of hydrated material in subduction zones as far back as 4.4 Ga (as recently suggested by [7]; and [8]). Instead, we propose that initiation of terrestrial subduction occurred at ca. 3.75 Ga, at which stage most of the Hadean basaltic shell (and its differentiation products) was recycled into the mantle, because of the lack of a stabilising mantle lithosphere. We further argue that >3.75 Ga terrestrial rocks and minerals were not preserved by chance, but because of creation of a lithospheric mantle keel concommitant with intrusion of voluminous granitoids immediately after establishment of global subduction. In other words, the only portions of >3.75 Ga crust (basaltic and otherwise) that survived were those that were involved in voluminous arc magmatism along the earliest subduction zones. [1] Nutman A.P. et al. (1999). Contr. Min. Pet. 137, 364. [2] Moorbath S. et al. (1973). Nature 245, 138. [3] Kamber B. S. et al.. (2001). Geol. Soc. London, Spec. Publ. 190, 177. [4] Frei R. & Rosing M. T. (in press). Chem. Geol. [5] Amelin Y. et al. (2000). GCA 64, 4205. [6] Scherer E. et al (2001) Science 293, 683. [7] Wilde S. A. et al.(2001). Nature 409, 175. [8] Mojzsis S. J. (2001). Nature 409, 178.

Magmatic tempo of Earth's youngest exposed plutons as revealed by detrital zircon U-Pb geochronology.

PubMed

Ito, Hisatoshi; Spencer, Christopher J; Danišík, Martin; Hoiland, Carl W

2017-09-29

Plutons are formed by protracted crystallization of magma bodies several kilometers deep within the crust. The temporal frequency (i.e. episodicity or 'tempo') of pluton formation is often poorly constrained as timescales of pluton formation are largely variable and may be difficult to resolve by traditional dating methods. The Hida Mountain Range of central Japan hosts the youngest exposed plutons on Earth and provides a unique opportunity to assess the temporal and spatial characteristics of pluton emplacement at high temporal resolution. Here we apply U-Pb geochronology to zircon from the Quaternary Kurobegawa Granite and Takidani Granodiorite in the Hida Mountain Range, and from modern river sediments whose fluvial catchments include these plutons in order to reconstruct their formation. The U-Pb data demonstrate that the Kurobegawa pluton experienced two magmatic pulses at ~2.3 Ma and ~0.9 Ma; whereas, to the south, the Takidani pluton experienced only one magmatic pulse at ~1.6 Ma. These data imply that each of these magmatic systems were both spatially and temporally distinct. The apparent ~0.7 Myr age gap between each of the three magmatic pulses potentially constrains the recharge duration of a single pluton within a larger arc plutonic complex.

Evaluating the utility of detrital thermochronometric studies: detrital laser ablation (U-Th)/He dating and conventional bedrock zircon (U-Th)/He analyses from the eastern Sierra Nevada, California

NASA Astrophysics Data System (ADS)

Horne, A.; Hodges, K. V.; Van Soest, M. C.

2016-12-01

Recent applications of the newly developed `laser ablation double dating' (LADD) technique, an integrated laser microprobe U/Pb and (U-Th)/He dating method, have showcased the potential utility of LADD for detrital thermochronologic studies. However, detrital thermochronologic techniques rely on confidence that detrital data adequately represent the full range of bedrock cooling ages within a catchment. To test this primary assumption, we compare (U-Th)/He zircon ages from age-elevation transects to LADD (U-Th)/He zircon ages from modern fluvial detritus collected at the range front in the eastern Sierra Nevada, California. Terminated by a normal fault escarpment, the small, steep catchments along the eastern side of the Sierra Nevada batholith are apropos locations for comparing the ability of detrital data to deduce the exhumation history of a source terrain with standard age-elevation transects. Additionally, the exhumation of the Sierra Nevada batholith is also intriguing, as past evaluations of the post-emplacement exhumation history of the range have yielded discrepant results. Thus far, analyses from the southern extent of the eastern Sierra Nevada show narrow ranges of cooling ages consistent with simple, relatively rapid exhumation. Ongoing analyses will expand the dataset such that we can fully compare bedrock and detrital age ranges as well as characterize the exhumation history of the range with a thermochronometer that has not been used to date the batholith.

Geochronology, geochemistry and Sr-Nd-Pb-Hf isotopes of the Early Jurassic granodiorite from the Sankuanggou intrusion, Heilongjiang Province, Northeastern China: Petrogenesis and geodynamic implications

NASA Astrophysics Data System (ADS)

Deng, Ke; Li, Qiugen; Chen, Yanjing; Zhang, Cheng; Zhu, Xuefeng; Xu, Qiangwei

2018-01-01

Mesozoic granitoid rocks represent a volumetrically component of the Northeastern (NE) China and preserve useful information about the tectonomagmatic history of this region. The Sankuanggou intrusion associated with skarn Fe-Cu deposit in the Duobaoshan ore field within NE China primarily consists of granodiorite with minor alkali-feldspar granite and diorite, which intrudes the Ordovician Duobaoshan Formation in the region. Zircon LA-ICP-MS U-Pb geochronology and whole-rock geochemistry, and Sr-Nd-Pb-Hf isotope analysis were performed on the Sankuanggou intrusion to investigate the petrogenesis and geodynamic implications. Zircon U-Pb dating of magmatic zircons from the granodiorite rock suggests that the intrusion was emplaced in the Early Jurassic (177 ± 1 Ma). Geochemically, it belongs to the metaluminous to slightly peraluminous high-K calc-alkaline I-type granitoids with a narrow range of SiO2 concentration (65.73-67.33 wt.%), high Ba, Sr, LREE and LILE contents and low abundance of Rb, Y, HREE and HFSE. All of these studied samples have homogeneous initial isotope traits with (87Sr/86Sr)i ranging from 0.70415 to 0.70423, εNd(t) of + 3.6 to + 4.0, (206Pb/204Pb)i = 17.933-18.458, (207Pb/204Pb)i = 15.520-15.587 and (208Pb/204Pb)i = 37.523-38.087, and zircon εHf(t) values varying from + 4.8 to + 9.9. These results, combined with the previous data, demonstrate that the Sankuanggou granitoids were formed by partial melting of the pre-existing juvenile crust in an extensional regime related to the post-collisional setting following the closure of the CAOB rather than previously proposed continental arc setting related to Paleo-Pacific or the Mongol-Okhotsk subduction, although their potential influence should not be dismissed.

Significance of zircon U-Pb ages from the Pescadero felsite, west-central California coast ranges

USGS Publications Warehouse

McLaughlin, Robert J.; Moore, Diane E.; ,; Martens, UWE C.; Clark, J.C.

2011-01-01

Weathered felsite is associated with the late Campanian–Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio–Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ∼185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ± prehnite ± laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefly Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86–90 Ma. Reflecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio–Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ∼100 km to the east in the Diablo Range–San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous forearc units of the Transverse Ranges. Based on zircon U-Pb ages, geologic and petrographic relations, the Pescadero felsite and a capping, sheared metaconglomerate underlie the Pigeon Point Formation. We infer that the magma formed by anatexis of Franciscan or Great Valley clastic sedimentary rocks originating from a parental Mesozoic Sierran-Mojave-Salinian calc-alkaline arc. The felsite erupted during Late Cretaceous time, was metamorphosed to pumpellyite-prehnite grade within the subduction zone, and then was rapidly exhumed, weakly zeolitized, and exposed before Pigeon Point forearc deposition. Pescadero volcanism apparently reflects a previously unrecognized ca. 86–90 Ma felsic igneous event in the accretionary margin.

Zircon U-Pb age of the Pescadero felsite: A late Cretaceous igneous event in the forearc, west-central California Coast Ranges

USGS Publications Warehouse

Ernst, W.G.; Martens, U.C.; McLaughlin, R.J.; Clark, J.C.; Moore, Diane E.

2011-01-01

Weathered felsite is associated with the late Campanian-Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio-Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ~185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ?? prehnite ?? laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe-reverse geometry (SHRIMPRG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefl y Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86-90 Ma. Refl ecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio-Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ~100 km to the east in the Diablo Range- San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous forearc units of the Transverse Ranges. Based on zircon U-Pb ages, geologic and petrographic relations, the Pescadero felsite and a capping, sheared metaconglomerate underlie the Pigeon Point Formation. We infer that the magma formed by anatexis of Franciscan or Great Valley clastic sedimentary rocks originating from a parental Mesozoic Sierran-Mojave-Salinian calcalkaline arc. The felsite erupted during Late Cretaceous time, was metamorphosed to pumpellyite-prehnite grade within the subduction zone, and then was rapidly exhumed, weakly zeolitized, and exposed before Pigeon Point forearc deposition. Pescadero vol canism apparently reflects a previously unrecognized ca. 86-90 Ma felsic igneous event in the accretionary margin. ?? 2011 Geological Society of America.

Evolution Of An Upper Crustal Plutonic-Volcanic Plumbing System:Insights From High Precision U-Pb Zircon Geochronology Of Intracaldera Tuff And Intrusions In Silver Creek Caldera, Arizona, USA

NASA Astrophysics Data System (ADS)

Zhang, T.; Mundil, R.; Miller, C. F.; Miller, J. S.; Paterson, S. R.

2010-12-01

Study of both plutonic and volcanic regimes in one single magmatic system is a powerful approach towards obtaining a more complete view of the long-term evolution of magma systems. The recently discovered Silver Creek caldera is the source of the voluminous Peach Spring Tuff (PST) (Ferguson, 2008) and presents a unique opportunity to study a field laboratory of a linked plutonic-volcanic system. This relict west-facing half caldera is predominantly filled with trachytic intracaldera tuff with the caldera margin intruded by several petrologically distinct hypabyssal intrusions. These include porphyritic granite with granophyric texture, felsic leucogranite, porphyritic monzonite exposed on NE side of the caldera that is zoned from more felsic to more mafic, and quartz-phyric dikes that intrude the caldera fill. We present preliminary single zircon ages from 4 samples that have been analyzed using the CA-TIMS method after thermal annealing and chemical leaching (Mattinson 2005), including 1 sample from intracaldera tuff and 3 samples from caldera-related intrusions. 3-D total U/Pb isochron ages from all four samples fall within a range of 18.32-18.90 Ma with uncertainties between 0.09 and 0.39 Ma, although some of them lack precision and are compromised by elevated common Pb. For example, zircon from the dated porphyritic monzonite yields an age of 18.32±0.42 Ma (MSWD=2.7) where the excess scatter may result from real age dispersion and/or different compositions of the common Pb contribution. The PST had been dated to ~18.5 Ma by 40Ar/39Ar techniques (Nielson et al., 1990). In order to be compared to U/Pb ages the 40Ar/39Ar age must be adjusted for a revised age for the then used flux monitor (MMbh-1) and corrected for the now quantified systematic bias between 40Ar/39Ar and U/Pb ages (Renne et al., 2010), which results in a corrected age of 18.8 Ma. Thus, the ages for our samples match that of the PST within error. Based on current results, the age difference between the different phases of the intrusion is very small and the ages of the intrusion match within errors the age of the PST. This tight time range indicates that the super-eruption and the subsequent reactivation of the caldera by hypabyssal intrusions happened on a much shorter timescale than the evolution of large magma systems that have been described with durations of up to 10 m.y. Additional geochronology in combination with geochemical and AMS analyses are aimed at a more detailed reconstruction of the emplacement and eruption history of this plutonic-volcanic system.

Post Flood Report March/April 1987.

DTIC Science & Technology

1987-04-01

JUSTIICATION ELECTE _ (SJUN .3 U DISIRIBWEION / NSRCTIEDo / AVAILABILIT CODES DISr AVAIL AND/OR SPECIALHOORHT___ETADREUNTD_-_____ DATE ACCESSIONED...Cotter - Secretary Appreciation is extended to Lawrence Bergen, Chief, Water Control Branch, who gave valuable assistance to RCC during the flood. Cover...photo of spillway discharge at Knightville Dam taken by Leo Milette on 6 April 1987 at 1320 hours with 1.9 feet of water over the crest. V.- V 8 8 4*1

Evolution of mare basalts - The complexity of the U-Th-Pb system

NASA Technical Reports Server (NTRS)

Unruh, D. M.; Tatsumoto, M.

1977-01-01

An attempt has been made to gain more insight into mare-basalt evolution by performing a very detailed leaching and mineral-separation U-Th-Pb systematics study on mare basalt 15085. It is found that about 20-50% of the U, Th, and Pb reside on the grain boundaries or in the mesostasis and that the Pb-207/Pb-206 ratios of the grain boundaries and crystal interiors are distinctly different. These distinct trends appear to represent either continuous or episodic postcrystallizational disturbances to the U-Th-Pb system of this rock. Using U and Pb partition coefficients, it is concluded that existing two- and three-stage U-Pb evolution models do not accurately describe mare-basalt genesis. An alternative two-stage + KREEP mixing model is proposed as a simple approximation to U-Pb evolution in lunar rocks. Most Rb-Sr and Sm-Nd data are compatible with this model.

U–Pb, Rb–Sr, and U-series isotope geochemistry of rocks and fracture minerals from the Chalk River Laboratories site, Grenville Province, Ontario, Canada

USGS Publications Warehouse

Neymark, Leonid; Peterman, Zell E.; Moscati, Richard J.; Thivierge, R. H.

2013-01-01

As part of the Geologic Waste Management Facility feasibility study, Atomic Energy of Canada Ltd. (AECL) is evaluating the suitability of the Chalk River Laboratories (CRL) site in Ontario, situated in crystalline rock of the southwestern Grenville Province, for the possible development of an underground repository for low- and intermediate-level nuclear waste. This paper presents petrographic and trace element analyses, U–Pb zircon dating results, and Rb–Sr, U–Pb and U-series isotopic analyses of gneissic drill core samples from the deep CRG-series characterization boreholes at the CRL site. The main rock types intersected in the boreholes include hornblende–biotite (±pyroxene) gneisses of granitic to granodioritic composition, leucocratic granitic gneisses with sparse mafic minerals, and garnet-bearing gneisses with variable amounts of biotite and/or hornblende. The trace element data for whole-rock samples plot in the fields of within-plate, syn-collision, and volcanic arc-type granites in discrimination diagrams used for the tectonic interpretation of granitic rocks.Zircons separated from biotite gneiss and metagranite samples yielded SHRIMP-RG U–Pb ages of 1472 ± 14 (2σ) and 1045 ± 6 Ma, respectively, in very good agreement with widespread Early Mesoproterozoic plutonic ages and Ottawan orogeny ages in the Central Gneiss Belt. The Rb–Sr, U–Pb, and Pb–Pb whole-rock errorchron apparent ages of most of the CRL gneiss samples are consistent with zircon U–Pb age and do not indicate substantial large-scale preferential element mobility during superimposed metamorphic and water/rock interaction processes. This may confirm the integrity of the rock mass, which is a positive attribute for a potential nuclear waste repository. Most 234U/238U activity ratios (AR) in whole rock samples are within errors of the secular equilibrium value of one, indicating that the rocks have not experienced any appreciable U loss or gain within the past 1 Ma. However, 234U/238U AR in fracture mineral samples collected down to borehole lengths of about 740 m deviate from the secular equilibrium value and 234U/238U model ages calculated for fracture mineral samples showing excess 234U range from 593 to 1415 ka, thus providing evidence of fracture flow in the associated bedrock during the past 1.5 Ma. Rare earth element patterns are variable in fracture-filling calcites and Fe oxides/hydroxides but are similar to those observed in associated whole-rock samples. The observed Ce anomalies are very small (CeN/CeN∗≈1">CeN/CeN∗≈1), do not vary with depth, and, therefore, do not contain evidence that the studied fracture minerals precipitated from oxidizing waters at the conceptual depth of a repository.

Comparative use of TIMS and SHRIMP for U Pb zircon dating of A-type granites and mafic tholeiitic layered complexes and dykes from the Corsican Batholith (France)

NASA Astrophysics Data System (ADS)

Cocherie, A.; Rossi, Ph.; Fanning, C. M.; Guerrot, C.

2005-05-01

The Corsica-Sardinia batholith in the southern realm of the Hercynian belt of Europe shows evidence for gravitational collapse of this part of the mountain belt, causing major felsic and mafic magmatism. The latest intrusions are composed of leucomonzogranite and late metaluminous and alkaline granite, associated with tholeiitic layered complexes and dykes. Three dating methods on zircon (Pb-evaporation, ID-TIMS and SHRIMP) were used to unravel the chronology of these felsic and mafic rocks. Dating of zircons by the conventional U-Pb method, using TIMS after zircon dissolution, achieved an analytical uncertainty of 1 Ma for favourable cases. The TIMS Pb-evaporation technique resulted in ages with an uncertainty range of 4 to 8 Ma. After 15 to 20 analyses with the SHRIMP method, a precision ranging from 2 to 5 Ma was obtained (all at 2 σ). The three methods applied to the same zircon population extracted from four A-type granites, show that the uncertainty ranges within 2-5 Ma according to the sample. This error seems to correspond to the real geochronological uncertainty that can be achieved. The results obtained show that all six tested alkaline granites were emplaced during a very short interval of about 3-5 Ma at about 288 Ma, almost contemporaneous with the leucomonzogranite emplacement (291-287 Ma) that ended the batholith formation. In addition, there is no significant gap with the age of emplacement of the mafic tholeiitic magmatism (around 286 Ma) crosscutting the "A-type" granites. The late alkaline granites definitely do not show up here as precursors of the Tethyan rifting that began at about 170 Ma, i.e. some 100 Ma after their emplacement. It is thus necessary to examine if alternative hypotheses to the anorogenic model of the A-type Younger Granite province better fit the new geochronological data. A model involving depleted continental-crust derived magma should be compatible with the timing and geodynamical constraints as far as isotopic data are concerned.

Is Myanmar jadeitite of Jurassic age? A result from incompletely recrystallized inherited zircon

NASA Astrophysics Data System (ADS)

Yui, Tzen-Fu; Fukoyama, Mayuko; Iizuka, Yoshiyuki; Wu, Chao-Ming; Wu, Tsai-Way; Liou, J. G.; Grove, Marty

2013-02-01

Zircons from two Myanmar jadeitite samples were separated for texture, mineral inclusion, U-Pb dating and trace element composition analyses. Three types of zircons, with respect to U-Pb isotope system, were recognized. Type I zircons are inherited ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons are metasomatic/hydrothermal ones, giving a (minimum) jadeitite formation age of 77 ± 3 Ma; and Type III zircons are incompletely recrystallized ones, with non-coherent and geologically meaningless ages from 153 to 105 Ma. These Myanmar jadeitites would therefore have formed through whole-sale metasomatic replacement processes. Compared with Type I zircons, Type II zircons show typical metasomatic/hydrothermal geochemical signatures, with low Th/U ratio (< 0.1), small Ce anomaly (Ce/Ce* = < 5) and low ΣREE content (40-115 ppm). Type III zircons, however, commonly have the above geochemical signatures straddle in between Type I and Type II zircons. It is shown that the resetting rates of various trace element compositions and U-Pb isotope system of inherited zircons are not coupled "in phase" in response to zircon recrystallization during jadeitite formation. The observed abnormally low Th/U ratio and small Ce anomaly of some Type I zircons, as well as the lack of negative Eu anomaly of all Type I zircons, should be suspected to be of secondary origin. In extreme cases, incompletely recrystallized zircons may show typical metasomatic/hydrothermal geochemical signatures, but leave U-Pb isotope system partially reset or even largely unchanged. Such zircons easily lead to incorrect age interpretation, and hence erroneous geological implication. The Myanmar jadeitites, based on the present study, might have formed during the Late Cretaceous subduction before the beginning of India-Asia continental collision at Paleocene. Previously proposed Late Jurassic ages for Myanmar jadeitites are suggested as results rooted on data retrieved from incompletely recrystallized inherited zircons.

U-Th-Pb systematics of some Apollo 17 lunar samples and implications for a lunar basin excavation chronology

NASA Technical Reports Server (NTRS)

Nunes, P. D.; Tatsumoto, M.; Unruh, D. M.

1974-01-01

U, Th, and Pb concentrations and lead isotopic compositions of selected Apollo 17 soil and rock samples are presented. Concordia treatments of U-Pb whole samples of Apollo 17 mare basalts and highland rocks probably reflect several early thermal events about 4.5 b.y. old more consistently than do U-Pb ages of samples collected at other lunar sites. We propose that all lunar U-Th-Pb data reflect a multistate U-Pb evolution history most easily understood as being related to a complex planetesimal bombardment history of the moon which apparently dominated lunar events from about 4.5 to about 3.9 b.y. ago. Semi-distinct events at about 4.0, about 4.2, and 4.4-4.5 b.y. are evident on whole-rock frequency versus Pb-207/Pb-206 age histograms. Each of these events may reflect multiple cratering episodes. For mare basalts, complete resetting of the source rock U-Pb systems owing to Pb loss relative to U was apparently often approached after a major planetesimal impact.

Geological Structure of the Basement of Western and Eastern Parts of the West-Siberian Plain

ERIC Educational Resources Information Center

Ivanov, Kirill S.; Erokhin, Yuriy V.; Ponomarev, Vladimir S.; Pogromskaya, Olga E.; Berzin, Stepan V.

2016-01-01

The U-Pb dating (SHRIMP-II on zircon) was obtained for the first time from the basement of the West Siberian Plain in the Western half of the region. It is established that a large part of the protolith of the metamorphic depth in the Shaim-Kuznetsov meganticlinorium contained sedimentary late- and middle-Devonian rocks (395-358 million years).…

P- T- t constraints on the development of the Doi Inthanon metamorphic core complex domain and implications for the evolution of the western gneiss belt, northern Thailand

NASA Astrophysics Data System (ADS)

Macdonald, A. S.; Barr, S. M.; Miller, B. V.; Reynolds, P. H.; Rhodes, B. P.; Yokart, B.

2010-01-01

The western gneiss belt in northern Thailand is exposed within two overlapping Cenozoic structural domains: the extensional Doi Inthanon metamorphic core complex domain located west of the Chiang Mai basin, and the Mae Ping strike-slip fault domain located west of the Tak batholith. New P- T estimates and U-Pb and 40Ar/ 39Ar age determinations from the Doi Inthanon domain show that the gneiss there records a complex multi-stage history that can be represented by a clockwise P- T- t path. U-Pb zircon and titanite dating of mylonitic calc-silicate gneiss from the Mae Wang area of the complex indicates that the paragneissic sequence experienced high-grade, medium-pressure metamorphism (M1) in the Late Triassic - Early Jurassic (ca. 210 Ma), in good agreement with previously determined zircon ages from the underlying core orthogneiss exposed on Doi Inthanon. Late Cretaceous monazite ages of 84 and 72 Ma reported previously from the core orthogneiss are attributed to a thermal overprint (M2) to upper-amphibolite facies in the sillimanite field. U-Pb zircon and monazite dating of granitic mylonite from the Doi Suthep area of the complex provides an upper age limit of 40 Ma (Late Eocene) for the early stage(s) of development of the actual core complex, by initially ductile, low-angle extensional shearing under lower amphibolite-facies conditions (M3), accompanied by near-isothermal diapiric rise and decompression melting. 40Ar/ 39Ar laserprobe dating of muscovite from both Doi Suthep and Doi Inthanon provided Miocene ages of ca. 26-15 Ma, representing cooling through the ca. 350 °C isotherm and marking late-stage development of the core complex by detachment faulting of the cover rocks and isostatic uplift of the sheared core zone and mantling gneisses in the footwall. Similarities in the thermochronology of high-grade gneisses exposed in the core complex and shear zone domains in the western gneiss belt of northern Thailand (and also in northern Vietnam, Laos, Yunnan, and central Myanmar) suggest a complex regional response to indentation of Southeast Asia by India.

The Wall-Rock Record of Incremental Emplacement in the Little Cottonwood-Alta Magmatic and Hydrothermal System, Wasatch Mountains, Utah, U.S.A.

NASA Astrophysics Data System (ADS)

Stearns, M.; Callis, S.; Beno, C.; Bowman, J. R.; Bartley, J. M.

2017-12-01

Contact aureoles record the cumulative effects on wall rocks of magma emplacement. Like the plutons they surround, contact aureoles have long been regarded to form geologically instantaneously. Protracted incremental emplacement of plutons must be reconciled with the wall-rock record of heat and mass transfer. Fundamental questions include how heat and material move from intrusions into their aureoles and how long that process takes. The Little Cottonwood stock is surrounded by a 2 km-wide contact aureole that contains prograde AFM mineral assemblages in the pelitic layers of the Proterozoic Big Cottonwood Formation. The Alta stock is surrounded by a well characterized 1 km-wide contact aureole containing both prograde AFM and CMS mineral assemblages in Ophir Shale and Mississippian dolostones, respectively. Understanding the petrogenesis of these aureoles requires the timing of magmatism and wall-rock metamorphism to be independently determined. Preliminary petrochronology (U/Th-Pb dates and trace element concentrations collected by LASS-ICP-MS) from the inner aureoles of both intrusions establishes a protracted history of monazite (re)crystallization from 35-25 Ma in the Little Cottonwood aureole and 35 Ma in the Alta aureole. Little Cottonwood aureole monazites are characterized by a positive age correlation with heavy rare earth elements (HREE) and a negative correlation with Eu/Eu*. Alta aureole monazites have a similar range of the HREE concentrations and Eu/Eu* variation. Zircon growth interpreted to record emplacement-level magmatic crystallization of the western Little Cottonwood stock ranges from 33-28 Ma near the contact. Multi-grain U-Pb zircon TIMS dates from the Alta stock range from 35-33 Ma and are interpreted to suggest the full range of emplacement-level magmatism in the Alta stock. Additionally, in situ U-Pb titanite dates from the Alta stock record intermittent high temperature hydrothermal activity in the stock margin from 35-24 Ma. These new data suggest that the Little Cottonwood aureole developed over several million years and overlapped in time with hydrothermal (re)crystallization of titanite within the Alta Stock. Both systems continued to develop after monazite (re)crystallization within Ophir Shale, which was concurrent with emplacement of the Alta Stock.

Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

USGS Publications Warehouse

Amato, J.M.; Toro, J.; Miller, E.L.; Gehrels, G.E.; Farmer, G.L.; Gottlieb, E.S.; Till, A.B.

2009-01-01

The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are similar to those in the peri-Gondwanan Avalonian-Cadomian arc system, the Timanide orogen of Baltica, and other circum-Arctic terranes that were proximal to Arctic Alaska prior to the opening of the Amerasian basin in the Early Cretaceous. Our Neoproterozoic reconstruction places the Arctic Alaska-Chukotka terrane in a position near Baltica, northeast of Laurentia, in an arc system along strike with the Avalonian-Cadomian arc terranes. Previously published faunal data indicate that Seward Peninsula had Siberian and Laurentian links by Early Ordovician time. The geologic links between the Arctic Alaska-Chukotka terrane and eastern Laurentia, Baltica, peri-Gondwanan arc terranes, and Siberia from the Paleoproterozoic to the Paleozoic help to constrain paleogeographic models from the Neoproterozoic history of Rodinia to the Mesozoic opening of the Arctic basin. ?? 2009 Geological Society of America.

The Grand St Bernard-Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U-Pb Dating

NASA Astrophysics Data System (ADS)

Bergomi, M. A.; Dal Piaz, G. V.; Malusà, M. G.; Monopoli, B.; Tunesi, A.

2017-12-01

The continental crust involved in the Alpine orogeny was largely shaped by Paleozoic tectono-metamorphic and igneous events during oblique collision between Gondwana and Laurussia. In order to shed light on the pre-Alpine basement puzzle disrupted and reamalgamated during the Tethyan rifting and the Alpine orogeny, we provide sensitive high-resolution ion microprobe U-Pb zircon and geochemical whole rock data from selected basement units of the Grand St Bernard-Briançonnais nappe system in the Western Alps and from the Penninic and Lower Austroalpine units in the Central Alps. Zircon U-Pb ages, ranging from 459.0 ± 2.3 Ma to 279.1 ± 1.1 Ma, provide evidence of a complex evolution along the northern margin of Gondwana including Ordovician transtension, Devonian subduction, and Carboniferous-to-Permian tectonic reorganization. Original zircon U-Pb ages of 371 ± 0.9 Ma and 369.3 ± 1.5 Ma, from calc-alkaline granitoids of the Grand Nomenon and Gneiss del Monte Canale units, provide the first compelling evidence of Late Devonian orogenic magmatism in the Alps. We propose that rocks belonging to these units were originally part of the Moldanubian domain and were displaced toward the SW by Late Carboniferous strike-slip faulting. The resulting assemblage of basement units was disrupted by Permian tectonics and by Mesozoic opening of the Alpine Tethys. Remnants of the Moldanubian domain became either part of the European paleomargin (Grand Nomenon unit) or part of the Adriatic paleomargin (Gneiss del Monte Canale unit), to be finally accreted into the Alpine orogenic wedge during the Cenozoic.

The North Patagonian orogenic front and related foreland evolution during the Miocene, analyzed from synorogenic sedimentation and U/Pb dating (˜42°S)

NASA Astrophysics Data System (ADS)

Ramos, Miguel E.; Tobal, Jonathan E.; Sagripanti, Lucía; Folguera, Andrés; Orts, Darío L.; Giménez, Mario; Ramos, Victor A.

2015-12-01

Miocene sedimentary successions of the Ñirihuau and Collón Cura formations east of the El Maitén Belt constitute a partial record of the Andean exhumation, defining a synorogenic infill of the Ñirihuau Basin in the foothills of the North Patagonian fold and thrust belt. Gravimetric and seismic data allow recognizing the internal arrangement and geometry of these depocenters that host both units, separating a synextensional section previous to the Andean development at these latitudes, from a series of syncontractional units above. A series of progressive unconformities in the upper terms shows the synorogenic character of these units corresponding to the different pulses of deformation that occurred during the middle Miocene. New U-Pb ages constrain these pulses to the ˜13.5-12.9 Ma interval and allow reconstructing the tectonic history of this region based on the detrital zircon source populations. The U-Pb maximum ages of sedimentation give to the Ñirihuau Formation in particular a younger age than previously assumed. Additionally, synsedimentary deformation in strata of the upper exposures of the Collón Cura Formation associated with contractional structures and U-Pb ages allow identifying a younger paleoseismogenic pulse in ˜11.3 Ma. Thus, based on these data and a compilation of previous datasets, a tectonic evolution is proposed characterized by a contractional episode that migrated eastwardly since ˜19 to 15 Ma producing the Gastre broken foreland and then retracted to the eastern North Patagonian Precordillera, where out-of-sequence thrusts cannibalized the wedge top zone in the El Maitén belt at ˜13.5-11.3 Ma.

Evaluating the provenance of Permian-Triassic and Palaeocene-Eocene ash beds by high precision U-Pb and Lu-Hf isotopic analyses of zircons: linking local sedimentary records to global events

NASA Astrophysics Data System (ADS)

Eivind Augland, Lars; Jones, Morgan; Planke, Sverre; Svensen, Henrik; Tegner, Christian

2016-04-01

Zircons are a powerful tool in geochronology and isotope geochemistry, as their affinity for U and Hf in the crystal structure and the low initial Pb and Lu allow for precise and accurate dating by U-Pb ID-TIMS and precise and accurate determination of initial Hf isotopic composition by solution MC-ICP-MS analysis. The U-Pb analyses provide accurate chronostratigraphic controls on the sedimentary successions and absolute age frames for the biotic evolution across geological boundaries. Moreover, the analyses of Lu-Hf by solution MC-ICP-MS after Hf-purification column chemistry provide a powerful and robust fingerprinting tool to test the provenance of individual ash beds. Here we focus on ash beds from Permian-Triassic and Palaeocene successions in Svalbard and from the Palaeocene-Eocene Thermal Maximum (PETM) in Fur, Denmark. Used in combination with whole rock geochemistry from the ash layers and the available geochemical and isotopic data from potential source volcanoes, these data are used to evaluate the provenance of the Permian-Triassic and Palaeocene ashes preserved in Svalbard and PETM ashes in Denmark. If explosive eruptions from volcanic centres such as the Siberian Traps and the North Atlantic Igneous Province (NAIP) can be traced to distal basins as ash layers, they provide robust tests of hypotheses of global synchronicity of environmental changes and biotic crises. In addition, the potential correlation of ash layers with source volcanoes will aid in constraining the extent of explosive volcanism in the respective volcanic centres. The new integrated data sets will also contribute to establish new reference sections for the study of these boundary events when combined with stable isotope data and biostratigraphy.

Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards

USGS Publications Warehouse

Black, L.P.; Kamo, S.L.; Allen, C.M.; Davis, D.W.; Aleinikoff, J.N.; Valley, J.W.; Mundil, R.; Campbell, I.H.; Korsch, R.J.; Williams, I.S.; Foudoulis, C.

2004-01-01

Precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) documentation is given for two new Palaeozoic zircon standards (TEMORA 2 and R33). These data, in combination with results for previously documented standards (AS3, SL13, QGNG and TEMORA 1), provide the basis for a detailed investigation of inconsistencies in 206Pb/238U ages measured by microprobe. Although these ages are normally consistent between any two standards, their relative age offsets are often different from those established by ID-TIMS. This is true for both sensitive high-resolution ion-microprobe (SHRIMP) and excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) dating, although the age offsets are in the opposite sense for the two techniques. Various factors have been investigated for possible correlations with age bias, in an attempt to resolve why the accuracy of the method is worse than the indicated precision. Crystallographic orientation, position on the grain-mount and oxygen isotopic composition are unrelated to the bias. There are, however, striking correlations between the 206Pb/238U age offsets and P, Sm and, most particularly, Nd abundances in the zircons. Although these are not believed to be the primary cause of this apparent matrix effect, they indicate that ionisation of 206Pb/238U is influenced, at least in part, by a combination of trace elements. Nd is sufficiently representative of the controlling trace elements that it provides a quantitative means of correcting for the microprobe age bias. This approach has the potential to reduce age biases associated with different techniques, different instrumentation and different standards within and between laboratories. Crown Copyright ?? 2004 Published by Elsevier B.V. All rights reserved.

Generation and Evolution of Quaternary Magmas Beneath Tengchong: Sr-Nd-Pb-Hf Isotope and Zircon U-series Age Constraints

NASA Astrophysics Data System (ADS)

Zou, H.; Ma, M.; Fan, Q.; Xu, B.; Li, S. Q.; Zhao, Y.; King, D. T., Jr.

2017-12-01

The Tengchong volcanic field on the southeastern margin of the Tibetan Plateau represents rare Quaternary volcanic eruptions on the plateau. The Quaternary Tengchong volcanic field formed high-potassium calc-alkaline volcanic rocks that include trachybasalts, basaltic trachyandesites, trachyandesites, and dacites. Herein, we present comprehensive Nd-Sr-Pb-Hf isotopic and elemental data for trachybasalts, basaltic trachyandesites, and trachyandesites from four young Tengchong volcanoes at Maanshan, Dayingshan, Heikongshan, and Laoguipo, in order to understand their magma genesis and evolution. Nd-Sr-Pb-Hf isotopes for the primitive Tengchong magma (trachybasalts with SiO2 <52.5 wt. % and MgO >5.5% wt. %) reflect a heterogeneous enriched mantle source. High Th/U, Th/Ta, and Rb/Nb ratios and Nd-Sr-Pb-Hf isotope characteristics of the primitive magmas suggest that the enriched mantle beneath Tengchong formed as a result of subduction of clay-rich sediments, which probably came from the Indian continental plate. Partial melting of the enriched mantle was generated by deep continental subduction coupled with recent regional extension in the Tengchong area. With regard to the evolved magmas (basaltic trachyandesites and trachyandesites), good correlations between SiO2 content and the ratios 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 177Hf/176Hf strongly suggest that the combined assimilation and fractional crystallization (AFC) was an important process during magma evolution to form these basaltic trachyandesites and trachyandesites. Uranium-series zircon dating on these evolved lavas from Tengchong is used to constrain their magma evolution and residence timescales.

Origin of lead from green glass of Apollo 15426: a search for primitive lunar lead.

USGS Publications Warehouse

Tatsumoto, M.; Premo, W.R.; Unruh, D.M.

1987-01-01

A major obstacle in lunar U-Pb chronology is the elusiveness of the primary Pb isotopic composition and U/Pb ratio and therefore the Pb evolution for the early history of the moon. In an attempt to seek the primitive lunar Pb isotopoc composition, green glass from lunar clod 15426,49 was studied for U-Th-Pb systematics because it is extremely Mg-rich and known to be the most primitive among sampled lunar volcanic rocks. Because of the low Pb concentration and high U/Pb ratio observed for the interior, the initial Pb was poorly defined. Nevertheless, the data indicate that lunar Pb evolved in an environment with 238U/204Pb = 19-55, which is considerably lower than those for mare basalts (around 300) but higher than values for the Earth (6-8).-from Authors

Zircon U-Th and U-Pb Ages From Quaternary Silicic Volcanic and Plutonic Rocks, and Their Bearing on Granitoid Batholiths

NASA Astrophysics Data System (ADS)

Bacon, C. R.

2007-12-01

In the ten years since publication of M. Reid et al.'s seminal paper on zircon ages from rhyolites (EPSL 150:2-39, 1997) >20 papers have appeared on SIMS 238U-230Th and 238U-206Pb geochronology of zircon from silicic volcanic rocks, plutonic xenoliths, and young intrusions. In some cases, as well as for U-Pb studies of Tertiary granitoids, plutonic samples are interpreted in the context of related volcanism. These geochronologic data have advanced conceptual models of silicic magma genesis and pluton construction. Of fundamental importance are discoveries that zircons in volcanic rocks typically pre-date eruption by 10's to 100's of kyr and that multiple zircon populations are common; these crystals are "antecrysts" recycled from intrusive rocks or crystal mush of the system that vented. Resolving such age differences is possible with U-Th at <300 ka but is challenging with U-Pb, where SIMS precision limits resolution of differences on the order of 100 kyr for Pleistocene-Miocene zircons. Cathodoluminescence (CL) imaging of polished crystals guides beam placement but leads to sampling bias that favors high-U regions. Thus, although model-age histograms and relative probability plots identify zircon age populations, they are unlikely to accurately define relative abundances of age groups. Microbeam analysis collects data for the entire volume sampled but only SIMS depth-profiling into crystal faces can spatially resolve fine zones. ID-TIMS analysis of CL-imaged zircon fragments can improve U-Pb precision. SIMS complements geochronology with trace element fingerprints of zircon growth environments and enables Ti-in-zircon thermometry. Literature examples illustrate recent findings: (1) rhyodacite lava at Crater Lake contains zircons derived from late Pleistocene granodiorite represented by blocks ejected in the caldera-forming eruption; (2) zircons in Mount St. Helens dacites grew at sub-eruption temperatures and pre-date eruptions by up to 250 kyr; (3) Miocene plutons near Mount Rainier and the Colorado River were emplaced and crystallized in pulses over ~2-3-Myr periods, some with coeval volcanics; and (4) Cretaceous batholiths in the Sierra Nevada and North Cascades preserve evidence of assembly over as much as 10 Myr; individual samples contain zircons that crystallized during intervals of >1 Myr. Zircon ages and wide-ranging trace element concentrations suggest crystallization mainly in differentiated melt pockets in high-crystallinity magmas that may repeatedly freeze and thaw. Some high-Th/U, incompatible-element rich, spongy textured zircons grew very late, in the presence of oxidizing fluid. Not all zircons survive recycling into undersaturated magmas, in which zircon will dissolve given enough time, depending on temperature and dissolved volatiles. Recent zircon geochronologic results for volcanic and plutonic rocks lend credence to the "mush model" of rhyolite genesis and batholith consolidation. Crystal-poor rhyolites and leucogranites are melts segregated by compaction or gas-driven filter pressing from granitoid crystal mush emplaced incrementally in the middle to upper crust and powered by basaltic magma repeatedly injected into the lower reaches of the mush column. Balance between heat loss and basaltic influx determines whether the mush freezes or partially thaws at any given time, blurs internal contacts in resulting plutons, and can produce large volumes of crystal-rich ignimbrite or rapid separation and eruption of crystal-poor rhyolite. Lifetimes of the largest volcano-plutonic systems, such as the Altiplano-Puna or Southern Rocky Mountains volcanic fields, are comparable to the ~10 Myr of the Tuolumne Intrusive Suite.

Automating U-Pb IDTIMS data reduction and reporting: Cyberinfrastructure meets geochronology

NASA Astrophysics Data System (ADS)

Bowring, J. F.; McLean, N.; Walker, J. D.; Ash, J. M.

2009-12-01

We demonstrate the efficacy of an interdisciplinary effort between software engineers and geochemists to produce working cyberinfrastructure for geochronology. This collaboration between CIRDLES, EARTHTIME and EarthChem has produced the software programs Tripoli and U-Pb_Redux as the cyber-backbone for the ID-TIMS community. This initiative incorporates shared isotopic tracers, data-reduction algorithms and the archiving and retrieval of data and results. The resulting system facilitates detailed inter-laboratory comparison and a new generation of cooperative science. The resolving power of geochronological data in the earth sciences is dependent on the precision and accuracy of many isotopic measurements and corrections. Recent advances in U-Pb geochronology have reinvigorated its application to problems such as precise timescale calibration, processes of crustal evolution, and early solar system dynamics. This project provides a heretofore missing common data reduction protocol, thus promoting the interpretation of precise geochronology and enabling inter-laboratory comparison. U-Pb_Redux is an open-source software program that provides end-to-end support for the analysis of uranium-lead geochronological data. The system reduces raw mass spectrometer data to U-Pb dates, allows users to interpret ages from these data, and then provides for the seamless federation of the results, coming from many labs, into a community web-accessible database using standard and open techniques. This EarthChem GeoChron database depends also on keyed references to the SESAR sample database. U-Pb_Redux currently provides interactive concordia and weighted mean plots and uncertainty contribution visualizations; it produces publication-quality concordia and weighted mean plots and customizable data tables. This initiative has achieved the goal of standardizing the data elements of a complete reduction and analysis of uranium-lead data, which are expressed using extensible markup language schema definition (XSD) artifacts. U-Pb_Redux leverages the freeware program Tripoli, which imports raw mass spectrometer data files and supports interactive review and archiving of isotopic data. Tripoli facilitates the visualization of temporal trends and scatter during measurement, statistically rigorous filtering of data and supports oxide and fractionation corrections. The Cyber Infrastructure Research and Development Lab for the Earth Sciences (CIRDLES) collaboratively integrates domain-specific software engineering with the efforts EARTHTIME and Earthchem. The EARTHTIME initiative pursues consensus-based approaches to geochemical data reduction, and the EarthChem initiative pursues the creation of data repositories for all geochemical data. CIRDLES develops software and systems for geochronology. This collaboration benefits the earth sciences by enabling geochemists to focus on their specialties using robust software that produces reliable results. This collaboration benefits software engineering by providing research opportunities to improve process methodologies used in the design and implementation of domain-specific solutions.

Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

NASA Astrophysics Data System (ADS)

Sauer, K. B.; Gordon, S. M.; Miller, R. B.; Vervoort, J. D.; Fisher, C. M.

2017-12-01

The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134-79 Ma). Sediment was likely underthrust into the active arc by circa 74-65 Ma, as soon as 7 Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79-60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.

Molecular Tracers of Saturated and Polycyclic Aromatic Hydrocarbon Inputs into Central Park Lake, New York City

PubMed Central

YAN, BEIZHAN; ABRAJANO, TEOFILO A.; BOPP, RICHARD F.; CHAKY, DAMON A.; BENEDICT, LUCILLE A.; CHILLRUD, STEVEN N.

2011-01-01

Saturated hydrocarbons (SH) and polycyclic aromatic hydrocarbons (PAHs) have been quantified in a sediment core obtained from Central Park Lake, New York City. Radionuclides 210Pb and 137Cs were used to assign approximate dates to each individual section in the core. The dating profile based on 210Pb matches very well with the time constraints provided by 137Cs. Radionuclide-derived depositional dates are consistent with temporal information from the petroleum-indicator ratio U/R [the ratio of unresolved complex mixture (UCM) to saturated hydrocarbons in the aliphatic fraction] and the history of fuel use in the NYC area. Ratios of 1,7-dimethylphenanthrane (DMP) to 1,7-DMP plus 2,6-DMP [1,7/(1,7 + 2,6)-DMP], retene to retene plus chrysene [Ret/(Ret + Chy)], and fluoranthene to fluoranthene plus pyrene [Fl/(Fl + Py)] provide additional source discrimination throughout the core. Results show that the ratio U/R is sensitive to petroleum inputs and Ret/(Ret + Chy) is responsive to contributions from softwood combustion, whereas both Fl/(Fl + Py) and 1,7/(1,7 + 2,6)-DMP can be used to discriminate among wood, coal, and petroleum combustion sources. Combined use of these ratios suggests that in New York City, wood combustion dominated 100 years ago, with a shift to coal combustion occurring from the 1900s to the 1950s. Petroleum use began around the 1920s and has dominated since the 1940s. PMID:16201624

Molecular tracers of saturated and polycyclic aromatic hydrocarbon inputs into Central Park Lake, New York City.

PubMed

Yan, Beizhan; Abrajano, Teofilo A; Bopp, Richard F; Chaky, Damon A; Benedict, Lucille A; Chillrud, Steven N

2005-09-15

Saturated hydrocarbons (SH) and polycyclic aromatic hydrocarbons (PAHs) have been quantified in a sediment core obtained from Central Park Lake, New York City. Radionuclides 210Pb and 137Cs were used to assign approximate dates to each individual section in the core. The dating profile based on 210Pb matches very well with the time constraints provided by 137Cs. Radionuclide-derived depositional dates are consistent with temporal information from the petroleum-indicator ratio U/R [the ratio of unresolved complex mixture (UCM)to saturated hydrocarbons in the aliphatic fraction] and the history of fuel use in the NYC area. Ratios of 1,7-dimethylphenanthrane (DMP) to 1,7-DMP plus 2,6-DMP [1,7/(1,7 + 2,6)-DMP], retene to retene plus chrysene [Ret/(Ret + Chy)], and fluoranthene to fluoranthene plus pyrene [FI/(FI + Py)] provide additional source discrimination throughoutthe core. Results show that the ratio U/R is sensitive to petroleum inputs and Ret/(Ret + Chy) is responsive to contributions from softwood combustion, whereas both FI/(FI + Py) and 1,7/ (1,7 + 2,6)-DMP can be used to discriminate among wood, coal, and petroleum combustion sources. Combined use of these ratios suggests that in New York City, wood combustion dominated 100 years ago, with a shift to coal combustion occurring from the 1900s to the 1950s. Petroleum use began around the 1920s and has dominated since the 1940s.

Detrital U-Pb zircon dating of lower Ordovician syn-arc-continent collision conglomerates in the Irish Caledonides

USGS Publications Warehouse

Clift, P.D.; Carter, A.; Draut, A.E.; Long, H.V.; Chew, D.M.; Schouten, H.A.

2009-01-01

The Early Ordovician Grampian Orogeny in the British Isles represents a classic example of collision between an oceanic island arc and a passive continental margin, starting around 480??Ma. The South Mayo Trough in western Ireland preserves a complete and well-dated sedimentary record of arc collision. We sampled sandstones and conglomerates from the Rosroe, Maumtrasna and Derryveeny Formations in order to assess erosion rates and patterns during and after arc collision. U-Pb dating of zircons reveals a provenance dominated by erosion from the upper levels of the Dalradian Supergroup (Southern Highland and Argyll Groups), with up to 20% influx from the colliding arc into the Rosroe Formation, but only 6% in the Maumtrasna Formation (~ 465??Ma). The dominant source regions lay to the northeast (e.g. in the vicinity of the Ox Mountains, 50??km distant, along strike). The older portions of the North Mayo Dalradian and its depositional basement (the Annagh Gneiss Complex) do not appear to have been important sources, while the Connemara Dalradian only plays a part after 460??Ma, when it supplies the Derryveeny Formation. By this time all erosion from the arc had effectively ceased and exhumation rates had slowed greatly. The Irish Grampian Orogeny parallels the modern Taiwan collision in showing little role for the colliding arc in the production of sediment. Negligible volumes of arc crust are lost because of erosion during accretion to the continental margin. ?? 2008 Elsevier B.V.

Advances and limitations on interpreting the erosional record from isotopic analysis of single detrital mineral grains

NASA Astrophysics Data System (ADS)

Parrish, Randall

2010-05-01

The analysis of provenance of clastic sediments is useful for reconstructing the characteristics and rates of exhumation of source areas, and sometimes placing minimum age constraints on depositional age. Due largely to increased availability and ease of access to LA-ICP-MS instrumentation, the analysis of provenance using single detrital accessory minerals has grown very rapidly over recent years. With this however is a culture of casual users who may not fully appreciate subtleties of measurement and isotope interpretation. The isotopic provenance literature is dominated by zircon-centric studies that use U-Pb dating and Hf isotope measurements of single zircons, but unfortunately an increasing number of these studies appear to lack sufficient understanding of U-Pb and Hf systematics; misleading interpretations are increasingly common. The inherent information contained in detrital accessory minerals is potentially immense, scientifically, but comprehensive interpretations attempting to reconstruct the geological make-up and evolution of sources require dating of multiple types of accessory minerals (i.e. zircon, titanite, monazite, garnet inclusions, micas, allanite, rutile, apatite) by various methods (U-Pb, fission track, Ar-Ar…) at times accompanied by isotope geochemical data (Lu-Hf, Sm-Nd, Rb-Sr) of phases where Sr, Hf, or REE comprise a major element (≥0.5%). Many approaches have been demonstrated but the mix of methodologies needs to be tailored to the problem, in view of the variable effort and expense needed to acquire good datasets. To date there are few comprehensive multi-mineral, multi-isotope system applications, and too many studies that follow a prescriptive cookbook that lacks innovation and fails to address a problem. The field needs to focus effort on the approaches that can solve a problem well rather than doing either just the easy methods or too many methods only moderately well. Zircon studies require strategies that reduce or eliminate discordance, collect sufficient data on each grain to make a robust age interpretation, improve accuracy of data by more attention to standards and uncertainties, can analyze thin overgrowths that reveal the magmatic or metamorphic age, and minimize sample consumption, not an easy task for the vast majority of laboratories doing provenance applications. Detrital monazite, monazite-in garnet, titanite and rutile can reveal much of the higher temperature metamorphic time-temperature path, and coupled U-Pb and fission track studies of single zircon and apatite grains can be useful for determining lower temperature exhumation rates. Isotope geochemistry (Hf-Nd-Sr-O) is more time consuming but can be pivotal to distinguish subtle differences in sources and to test specific hypotheses. Examples of improved methods and applications will be presented to illustrate the presentation.

ESR and mass-spectrometric uranium-series dating studies of a mammoth tooth from stanton harcourt, Oxfordshire, England

NASA Astrophysics Data System (ADS)

Zhou, L. P.; McDermott, F.; Rhodes, E. J.; Marseglia, E. A.; Mellars, P. A.

The age of the Channel Deposits at Stanton Harcourt, Oxfordshire, England, has been a topic of debate with important implications for British Pleistocene stratigraphy. Recent excavations led by K. Scott reveal ample evidence for ancient environmental conditions characteristic of an interglacial. However, the question remains on the assignment of its age. At present it is thought to represent an interglacial corresponding to either marine OI Stage 7 or 5e. In an attempt to constrain the chronology of the site, and to assess the techniques' reliability, we have made electron spin resonance (ESR) measurements on enamel and mass-spectrometric U-series measurements on both enamel and dentine from a mammoth tooth buried in the Channel Deposits at Stanton Harcourt. Four dentine samples gave U-series dates between 65.4±0.4 and 146.5±1.0 ka and two enamel samples between these dentine layers were dated to 53.3±0.2 and 61.1±0.6 ka. The corresponding ESR age estimates for the enamel samples are 59±6 and 62±4 ka (early U-uptake, EU) and 95±11 and 98±7 ka (linear U-uptake, LU). The recent U-uptake (RU) dates are 245±38 and 238±31 ka, but in light of the U-series data we would not expect these to represent realistic age estimates. Similar ESR results were obtained from two other adjacent enamel samples. The effect of the large size of the mammoth tooth on the external gamma dose, and the internal gamma contribution from the high U content of the dentine, are considered. While the recent uptake ESR dates appear to coincide with OI Stage 7, all the early and linear uptake ESR and mass-spectrometric U-series dates are younger than the expected age estimation based on recent geological interpretation and amino acid racemisation measurements (>200 ka) and optical dating studies (200-450 ka). Possible causes of the unexpected dating results are discussed. We conclude that our mass-spectrometric U-series and EU and LU ESR measurements represent minimum age estimates for the tooth analysed. Our results seem to suggest that the tooth and hence the Channel Deposits are at least 147 ka in age. i.e. predating the last interglacial.

U-Th-Pb isotope data indicate phanerozoic age for oxidation of the 3.4 Ga Apex Basalt

NASA Astrophysics Data System (ADS)

Li, Weiqiang; Johnson, Clark M.; Beard, Brian L.

2012-02-01

The occurrence of ferric oxides in Archean rocks has played an important role in discussions on the amount of free oxygen in the atmosphere of the ancient Earth. Recognizing that post-Archean weathering may also produce oxide minerals, drill cores have been used to obtain samples beneath the depth of Phanerozoic weathering. The first core of the Archean Biosphere Drilling Project (ABDP-1) documented hematite as alteration products in 3.4 Ga basalts from the Marble Bar area of the Pilbara Craton, NW Australia, and this has been used to infer the presence of an O2-bearing atmosphere in the Archean. It is possible, however, that despite recovery of samples from > 100 m depth, oxidation of the basalts occurred much younger than the depositional age. In this study, the age of oxidation of the Apex Basalt from the ABDP-1 drill core at Marble Bar is constrained by U-Th-Pb geochronology. Lead and U concentrations of the basalts from the ABDP-1 drill core vary greatly, between < 1-58 ppm and 0.08-1.04 ppm, respectively, whereas Th contents are more restricted (0.24-0.71 ppm). 206Pb/204Pb ratios are non-radiogenic and vary from 12.44 to 14.69. The linear array in terms of 206Pb/204Pb-207Pb/204Pb variations does not reflect an age but reflects two-component mixing between a non-radiogenic "ore lead" end member and a radiogenic "basalt lead" end member. The samples do not form isochrons on 238U/204Pb-206Pb/204Pb, 235U/204Pb-207Pb/204Pb, or 232Th/204Pb-208Pb/204Pb diagrams, indicating post-formation U and Pb addition. Comparison of measured U/Th ratios with "model" U/Th ratios calculated based on 208Pb/204Pb-206Pb/204Pb variations indicates that U enrichment most likely occurred in the last 200 Ma. The degree of U enrichment in the samples is correlated with Fe(III)/FeTotal ratios, indicating that U addition and oxidation were related, most likely reflecting penetration of oxygenated surface waters in the Phanerozoic along bedding planes and shear zones. These results, therefore, indicate that oxidation of the Apex Basalt did not occur in the Archean, and therefore cannot be used to infer an oxygenated atmosphere at that time.

Relict zircon U-Pb age and O isotope evidence for reworking of Neoproterozoic crustal rocks in the origin of Triassic S-type granites in South China

NASA Astrophysics Data System (ADS)

Gao, Peng; Zheng, Yong-Fei; Chen, Yi-Xiang; Zhao, Zi-Fu; Xia, Xiao-Ping

2018-02-01

Granites derived from partial melting of sedimentary rocks are generally characterized by high δ18O values and abundant relict zircons. Such relict zircons are valuable in tracing the source rocks of granites and the history of crustal anatexis. Here we report in-situ U-Pb ages, O isotopes and trace elements in zircons from Triassic granites in the Zhuguangshan and Jiuzhou regions, which are located in the Nanling Range and the Darongshan area, respectively, in South China. Zircon U-Pb dating yields magma crystallization ages of 236 ± 2 Ma for the Zhuguangshan granites and 246 ± 2 Ma to 252 ± 3 Ma for the Jiuzhou granites. The Triassic syn-magmatic zircons are characterized by high δ18O values of 10.1-11.9‰ in Zhuguangshan and 8.5-13.5‰ in Jiuzhou. The relict zircons show a wide range of U-Pb ages from 315 to 2185 Ma in Zhuguangshan and from 304 to 3121 Ma in Jiuzhou. Nevertheless, a dominant age peak of 700-1000 Ma is prominent in both occurrences, demonstrating that their source rocks were dominated by detrital sediments weathered from Neoproterozoic magmatic rocks. Taking previous results for regional granites together, Neoproterozoic relict zircons show δ18O values in a small range from 5 to 8‰ for the Nanling granites but a large range from 5 to 11‰ for the Darongshan granites. In addition, relict zircons of Paleozoic U-Pb age occur in the two granitic plutons. They exhibit consistently high δ18O values similar to the Triassic syn-magmatic zircons in the host granites. These Paleozoic relict zircons are interpreted as the peritectic product during transient melting of the metasedimentary rocks in response to the intracontinental orogenesis in South China. Therefore, the relict zircons of Neoproterozoic age are directly inherited from the source rocks of S-type granites, and those of Paleozoic age record the transient melting of metasedimentary rocks before intensive melting for granitic magmatism in the Triassic.

U-Pb zircon and biostratigraphic data of high-pressure/low-temperature metamorphic rocks of the Talea Ori: tracking the Paleotethys suture in central Crete, Greece

NASA Astrophysics Data System (ADS)

Zulauf, G.; Dörr, W.; Krahl, J.; Lahaye, Y.; Chatzaras, V.; Xypolias, P.

2016-10-01

Inherited deformation microfabrics of detrital quartz grains and U-Pb (Laser ablation (LA)-ICPMS and ID TIMS) ages of detrital zircons separated from the Phyllite-Quartzite Unit s.l. of the Talea Ori, central Crete, suggest strikingly different source rocks. Albite gneiss of the lower Rogdia Beds includes Cambrian and Neoproterozoic rounded zircons with main U-Pb age peaks at 628 and 988 Ma. These and minor Paleoproterozoic and Archean peaks, together with the lack of Variscan-aged and Mesoproterozoic zircons, are similar to the age spectra obtained from the Phyllite-Quartzite Unit s.str. of the Peloponnesus and eastern Crete and from the Taurides. All of these zircons should be derived from the northeastern passive margin of Gondwana (Cimmeria). Metatuffites of the uppermost Rogdia Beds and metasandstone of Bali beach, on the other hand, include euhedral detrital zircons displaying a Variscan U-Pb age spectra at ca. 300 Ma with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U-Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta)conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top-to-the-north shearing, which post-dates the growth of blue amphiboles (crossite).

U-Pb (SHRIMP) Ages of Be and U-rich Opal in Tuffaceous Breccia at Spor Mountain, Utah: Interpreting a Record of Continuous Opal Formation, Episodic Be-U Mineralization, and Remobilization Events

NASA Astrophysics Data System (ADS)

Ayuso, R. A.; Vazquez, J. A.; Foley, N.; Lederer, G.; Jaskula, B.

2016-12-01

The Spor Mountain Fm. (SMF, ca. 21 Ma; Lindsey, 1977, Eco. Geol., v. 72, 219-232; Foley et al., 2012, USGS SIR 2010-5070-F, 1-43) hosts the largest deposit of volcanogenic-epithermal Be in the world (proven reserves 15,700 t/contained Be). Ore occurs mainly in cm-to-m-wide irregularly layered nodules of calcite, chalcedony, opal, fluorite, and bertrandite (Be4Si2O7(OH)2) in tuff breccias. U-Pb SHRIMP dating (adapted from Paces et al., 2004, GCA v. 68, 1591-1606; Neymark and Paces, 2013, EPSL v. 361, 98-109) of opal in nodules from SMF yielded 206Pb/238U ages coupled with multi-element analyses, e.g., Be, F, P, Si, Ti, REE, etc. The ages reveal periods of prolonged massive and fracture-filling opal formation that range from 55 Ma to 2 Ma. Age gaps are not prominent (a previous study of bulk samples identified opal ages of 21.8 Ma, 13-16 Ma, 8-9 Ma in SMF and 3.5 Ma in the overlying 6 Ma Topaz Mountain Fm.; Ludwig et al., 1980, EPSL, v. 46, 221-232). High values of Be/Si ( 5,000-20,000), Be/F, Be/P, and Be/U and oldest ages ( 55 to 28 Ma) occur in opal in nodule cores; outward, younger opal layers ( 28 to 7 Ma) decline in Be/Si but also include spikes of >5,000. A prominent U/Si spike (>300) occurs in opal between 6 Ma and 4 Ma, which may establish the age of U mineralization that occurs immediately east of Spor Mountain (the Yellow Chief U deposit). The occurrence of Be-rich opal older than 25 Ma in nodules within the 21 Ma (K-Ar date) tuff suggests that nodule formation may also be associated with older volcanism in the region. Opal that is younger than 21 Ma is thought to have formed by hydrothermal fluid interacting with Be-rich tuff. Geochemical modeling shows leaching of Be and other elements from volcanic glass and deposition of bertrandite upon reaction of the fluid with carbonate clasts in the tuff are viable mechanisms for the observed assemblages. Be concentrations in late nodular opal (<6 Ma) may reflect redistribution of earlier mineralization. The world-class deposits at Spor Mountain likely formed by prolonged magmatic-hydrothermal processes that include multiple Be mineralization and remobilization events.

U-Pb age constraints for the La Tuna Granite and Montevideo Formation (Paleoproterozoic, Uruguay): Unravelling the structure of the Río de la Plata Craton

NASA Astrophysics Data System (ADS)

Pamoukaghlián, Karina; Gaucher, Claudio; Frei, Robert; Poiré, Daniel G.; Chemale, Farid; Frei, Dirk; Will, Thomas M.

2017-11-01

The Río de la Plata Craton is a continental block that crops out in Uruguay, eastern Argentina, southernmost Brazil and Paraguay. It comprises in Uruguay the Piedra Alta, Tandilia and Nico Pérez terranes, separated by the Colonia and the Sarandí del Yí megashears. The La Tuna Granite, which intrudes the Araminda metasandstones in the Tandilia Terrane, was considered Cambrian in age and the intruded sandstones were assigned to the Neoproterozoic Piedras de Afilar Formation. We show that the granite is Paleoproterozoic in age and that the host metasandstones do not belong to the Piedras de Afilar Formation, but to the Paleoproterozoic Montevideo Formation. U-Pb LA ICP-MS of zircon ages for the La Tuna Granite yielded a concordant crystallization age of 2156 ± 26 Ma. Furthermore a metamorphic event at 2010 ± 9 Ma is revealed by Pb stepwise leaching dating of monazites. U-Pb detrital zircon ages of the host Araminda metasandstone yield an upper intercept discordia age of 2152 ± 29 Ma, which marks the intrusion of the La Tuna pluton, and which is in accordance with the zircon U-Pb LA ICP MS constraints. A concordant U-Pb detrital zircon age of 2465 ± 40 Ma provides a maximum depositional age constraint for the metapsammites. Comparing quartz arenites of the Ediacaran Piedras de Afilar Formation with the Araminda metaquartzites, we conclude that they are very similar regarding petrology but they differ in age and metamorphic overprint. Detrital zircons in quartz arenites of the Piedras de Afilar Formation show youngest ages of 1.0 Ga. On the other hand, detrital zircons recovered from the Araminda metasandstones and the age of the intruding granite allow interpreting a depositional age between 2465 and 2150 Ma. Nd model ages show crustal residence times in average more than 200 myr older for the Tandilia Terrane both in Uruguay and Argentina, with a significant Neoarchean component, which is lacking in the Piedra Alta Terrane. Whereas the Piedra Alta Terrane was formed rapidly from a juvenile source, the Tandilia Terrane shows a more protracted history. Our data support the interpretation of the Tandilia Terrane as a separate tectonostratigraphic unit.

Coupled dissolution-precipitation in natural monazite: effect of irradiation damage or fluid mediation?

NASA Astrophysics Data System (ADS)

Seydoux-Guillaume, Anne-Magali; Montel, Jean-Marc; de Parseval, Philippe; Bingen, Bernard; Janots, Emilie

2010-05-01

The LREE orthophosphate monazite is a common accessory mineral, important as a U-Th-Pb geochronometer in both metamorphic and magmatic rocks. In order to correctly interpret measured ages, it is essential to properly understand mechanisms that control them. Few studies have shown that coupled dissolution-crystallisation in the presence of a fluid phase is a mechanism incomparably more efficient that solid state diffusion to reset isotopic signature within monazite grains. It is known that dissolution-precipitation is efficiency enhanced by the presence of defects within crystals. Because of its high actinide contents (U and Th), monazite receives intense self-irradiation doses. In contrast to zircon (a silicate), monazite (a phosphate) is less sensitive to irradiation. Natural amorphous monazite has never been reported and the only proof that monazite lattice was destroyed by irradiation is shown by the presence of lattice distortion (strained lattice); this is because defect healing is more efficient than amorphization. The present study focuses on large (cm) single monazite crystals from five distinct localities in Norway, Madagascar, Srilanka, Zwaziland and Morefield. They have different chemical compositions, especially with regard to U, Th and Pb contents, and have ages ranging from ca. 500 to 1000 Ma. Nevertheless, all of them share the same petrographic features. Optical microscope and SEM images reveal variably intense fracturation. BSE imaging in the SEM indicates that monazite is composed of multiple phases: an unaltered monazite (Mnz1) + an altered monazite (Mnz2) associated with Th-rich phase (Thorium silicate or Thorium oxide) +/- Xenotime, depending on the initial composition of Mnz1. Analogous textures were already described by Seydoux-Guillaume et al. (2007) and Hetherington and Harlov (2008;). The alteration textures are always associated with radial cracks emanating from the high radioactive phase (Th-rich phase). The question addressed in the discussion is the role and the chronology of each process, i.e. irradiation vs coupled dissolution-precipitation. U-Th-Pb ages obtained by chemical dating on electron microprobe from altered and unaltered monazites show no significant differences. Therefore U-Pb dating using SIMS and LA-ICP-MS are in progress to determine precise isotopic age that would refine the alteration chronology. Finally, these results will be compared with experimental work, which are currently investigating the role of structural defects on coupled dissolution-precipitation in monazite. Hetherington and Harlov (2008). Am. Mineral., 93, 806-820. Seydoux-Guillaume et al. (2007). Eur. J. Mineral., 19, 7-14.

Helium Diffusion in Natural Xenotime

NASA Astrophysics Data System (ADS)

Anderson, A. J.; Hodges, K. V.; Van Soest, M. C.; Hanchar, J. M.

2017-12-01

Xenotime (nominally YPO4) occurs as an accessory mineral in felsic igneous rocks, pegmatites, and gneisses, often incorporating weight percent levels of U, Th, and REEs. Although commonly used for precise U-Pb geochronology, xenotime is seldom used for (U-Th)/He thermochronometry despite the fact that its high radionuclide content allows for the rapid accumulation of radiogenic He that could potentially allow for the precise dating of very young cooling events in active geologic settings. Xenotime's likely high concentrations of radiogenic 4He also make it a promising candidate for laser microprobe (U-Th)/He dating. Unfortunately, limited data are currently available for He diffusion in xenotime, and previous experimental studies of natural and synthetic xenotime have led to discrepant results (1). Published estimates of the (U-Th)/He closure temperature for xenotime based on these experiments range from 60˚C to 300˚C. In this contribution, we report new results for incremental step heating `bulk' 4He diffusion experiments on a fragment of a natural xenotime crystal from the Torghar district of the Khyber Agency in the Federally Administered Tribal Areas of Pakistan. Laser ablation 238U/206Pb dating of this crystal yields a crystallization age of 28.82 ± 0.13 Ma. The results of our He diffusion experiments- which display excellent linearity on an Arrhenius plot assuming a spherical geometry - indicate kinetic parameters of E 133 kJ/mol and ln(D0/a2) 10.9 s-1. Our results imply a preliminary (U-Th)/He closure temperature of 80˚C for xenotime crystals with a diffusion dimension of 220 μm. This closure temperature is much lower than that previously estimated for natural xenotime (1), and it is possible, perhaps likely, that He diffusion in natural xenotime crystals display a strong compositional dependency (1). Nevertheless, our experimental data suggest that near-end member (YPO4) xenotime has an intrinsic helium diffusivity consistent with a closure temperature comparable to that of the apatite (U-Th)/He thermochronometer. As a consequence, xenotime He chronometry may serve as a higher-precision alternative to apatite He chronometry for revealing low temperature cooling histories of rocks in which it crystallized. References: 1) Farley, 2007, Geochimica et Cosmochimica Acta, v. 71.

Distribution, provenance, and onset of the Xiashu Loess in Southeast China with paleoclimatic implications

NASA Astrophysics Data System (ADS)

Wang, Xiaoyong; Lu, Huayu; Zhang, Hanzhi; Wu, Jiang; Hou, Xiaoxue; Fu, Yu; Geng, Junyan

2018-04-01

Loess deposits are important archives of past climate change in subtropical China, where long-term terrestrial records are scarce. However, only a few long-term records with reliable dating were reported in Jiangsu Province, where loess deposits in southeastern China were first discovered and designated as the Xiashu Loess. Moreover, the provenance of the Xiashu Loess is still controversial. This study presents the magnetostratigraphy and detrital zircon U-Pb age results of loess deposits at Zhoujiashan in Nanjing, Jiangsu Province. To date, this section (with basal age of approximately 0.88 Ma) may be the oldest loess deposits discovered in southeastern China. Based on the detrital zircon U-Pb ages of the Zhoujiashan section, together with published results of geochemistry, geomorphology, sedimentology, and meteorology, we propose that the Xiashu Loess has two mixed sources. Local materials derived from the Yangtze River Basin (including exposed river/lake beds, floodplains, and mountains) served as a major contributor. The distal eolian dust that originated from the arid areas of North China (including deserts, piedmont alluvial fans, and drylands) played a minor role in the formation process of the Xiashu Loess. The nearly synchronous onset age (approximately 0.88-0.85 Ma) of the Xiashu Loess was attributed to a regional environmental event during the middle Pleistocene transition.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis.

PubMed

Heimdal, Thea H; Svensen, Henrik H; Ramezani, Jahandar; Iyer, Karthik; Pereira, Egberto; Rodrigues, René; Jones, Morgan T; Callegaro, Sara

2018-01-09

The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO 2 . In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206 Pb/ 238 U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.

Zircon U-Pb age and Hf-O isotopes of felsic rocks from the Atlantis Bank, Southwest Indian Ridge

NASA Astrophysics Data System (ADS)

Liu, C. Z.; Zhang, W. Q.

2017-12-01

Hole U1473A was drilled to 790 meters below seafloor on the Atlantis Bank, an oceanic core complex in the Southwest Indian Ridge, where the upper crust has been removed by detachment faulting. The recovered core consists dominantly of olivine gabbro, with subordinate gabbro, gabbro with varying Fe-Ti oxide concentrations. Felsic veins intermittently occur throughout the whole core section. Zircons separated from twenty-four felsic samples have been conducted for U-Pb dating and O isotope analyses on the Cameca 1280 and Lu-Hf isotopes by laser ablation coupled with a MC-ICPMS. The zircons have highly variable contents of U (12-2078 ppm) and Th (5-801 ppm), yielding Th/U ratios of 0.33-0.81. They are typical oceanic zircons as defined by the trace element discrimination plots of Grimes et al. (2015). The weighted mean 206Pb/238U ages of the analyzed zircons vary from 11.29 to 12.57 Ma. Age differences between felsic veins throughout the whole core are not resolved within analytical uncertainty of the SIMS measurements. All felsic samples have similar zircon Hf isotope compositions, with initial 176Hf/177Hf ratios of 0.283126-0.283197 and ɛHf values of 12.76-15.27. Zircons from all felsic samples but one have mantle-like δ18O values of 5.14-5.50‰. Zircons from one sample show partial resorption or total recrystallization; in comparison, they have lower δ18O values of 4.81±0.21‰. Such characteristics provide clear evidence for hydrothermal alteration after magmatic intrusion.

Re-Os, Sm-Nd, U-Pb, and stepwise lead leaching isotope systematics in shear-zone hosted gold mineralization: genetic tracing and age constraints of crustal hydrothermal activity

NASA Astrophysics Data System (ADS)

Frei, R.; Nägler, Th. F.; Schönberg, R.; Kramers, J. D.

1998-06-01

A combined Re-Os, Sm-Nd, U-Pb, and stepwise Pb leaching (PbSL) isotope study of hydrothermal (Mo-W)-bearing minerals and base metal sulfides from two adjacent shear zone hosted gold deposits (RAN, Kimberley) in the Harare-Shamva greenstone belt (Zimbabwe) constrain the timing of the mineralizing events to two periods. During an initial Late Archean event (2.60 Ga) a first molybdenite-scheelite bearing paragenesis was deposited in both shear zone systems, followed by a local reactivation of the shear systems during an Early Proterozoic (1.96 Ga) tectono-thermal overprint, during which base metal sulfides and most of the gold was (re-)deposited. While PbSL has revealed an open-system behavior of the U-Pb systematics in molybdenite and wolframite from the RAN mine, initial Archean Re-Os ages are still preserved implying that this system in these minerals was more resistant to the overprint. A similar retentivity could be shown for the Sm-Nd system in scheelite and powellite associated with the above ore minerals. Re-Os isotopic data from the Proterozoic mineralization in the Kimberley mine point to a recent gain of Re, most pronouncedly affecting Fe-rich sulfides such as pyrrhotite. A significant Re-loss in powellitic scheelite (an alteration phase of molybdenite-bearing scheelite), coupled with a marked loss of U in W-Mo ore minerals, complements the observation of a major Re uptake in Fe-sulfides during oxidizing conditions in a weathering environment. Pyrrhotite under these conditions behaves as an efficient Re-sink. Lead isotope signatures from PbSL residues of molybdenite, powellite, and quartz indicate a continental crustal source and/or contamination for the mineralizing fluid by interaction of the fluids with older sedimentary material as represented by the direct host country rocks. Our investigation reveals the potential of the Re-Os isotopic system applied to crustal hydrothermal ore minerals for genetic tracing and dating purposes. The simplified chemical separation of Re and Os from geological material used in this study, together with improvements of chemical yields, will enable high precision data to be collected rapidly on crustal material with low Os concentrations in the future.

Nature of the Yucatan Block Basement as Derived From Study of Granitic Clasts in the Impact Breccias of Chicxulub Crater

NASA Astrophysics Data System (ADS)

Vera-Sanchez, P.; Rebolledo-Vieyra, M.; Perez-Cruz, L.; Urrutia-Fucugauchi, J.

2008-05-01

The tectonic and petrologic nature of the basement of the Yucatan Block is studied from analyses of basement clasts present in the impact suevitic breccias of Chicxulub crater. The impact breccias have been sampled as part of the drilling projects conducted in the Yucatan peninsula by Petroleos Mexicanos, the National University of Mexico and the Chicxulub Scientific Drilling Project. Samples analyzed come mainly from the Yaxcopoil-1, Tekax, and Santa Elena boreholes, and partly from Pemex boreholes. In this study we concentrate on clasts of the granites, granodiorites and quartzmonzonites in the impact breccias. We report major and trace element geochemical and petrological data, which are compared with data from the granitic and volcanic rocks from the Maya Mountains in Belize and from the Swannee terrane in Florida. Basement granitic clasts analyzed present intermediate to acidic sub-alkaline compositions. Plots of major oxides (e.g., Al2O3, Fe2O3, TiO2 and CaO) and trace elements (e.g., Th, Y, Hf, Nb and Zr) versus silica allow separation of samples into two major groups, which can be compared to units in the Maya Mountains and in Florida basement. The impact suevitic breccia samples have been affected by alteration likely related to the hydrothermal processes associated with the crater melt sheet. Cloritization, seritization and fenitization alterations are recognized, due to the long term hydrothermalism. Krogh et al. (1993) reported U-Pb dates on zircons from the suevitic breccias, which gave dates of 545 +/- 5 Ma and 418 +/- 6 Ma, which were interpreted in terms of the deep granitic metamorphic Yucatan basement. The younger date correlates with the age for the Osceola Granite and the St. Lucie metamorphic complex of the Swannee terrane in the Florida peninsula. The intrusive rocks in the Yucatan basement may be related to approx. 418 Ma ago collisional event in the Late Silurian.

Geochronology and Regional Correlation of Continental Permo-Triassic Sediments in West Texas

NASA Astrophysics Data System (ADS)

Mitchell, W.; Renne, P. R.; Mundil, R.; Chang, S.; Geissman, J. W.; Tabor, N. J.; Mack, G.

2011-12-01

Although many aspects of marine sections spanning the Permian-Triassic boundary (PTB) have been studied in great detail across a broad paleogeographic area, less is known about the timing, pace, and extent of environmental changes and extinctions across this boundary in continental environments, particularly along the Panthalassa margin. Extensive outcrops in the Ochoan Series of west Texas provide an opportunity to investigate the terrestrial record spanning the PTB. The presence of several silicic tuffs in these sections allows for precise radioisotopic dating using both U-Pb and 40Ar/39Ar techniques. Dated strata then serve as a calibration basis for paleomagnetic and lithostratigraphic studies and facilitate stratigraphic correlation across the few to hundreds of kilometers separating study sites. Depending on the possible correlations, as many as seven tuffs have been identified in this region, the ages of which are within about a million years of the chronometrically-defined PTB at the Meishan section in China at ca. 252 Ma. Data obtained thus far indicate that the PTB occurs within the Quartermaster/Dewey Lake Formation. With the aims of determining the number and ages of distinct tuffs found and facilitating a well-correlated regional stratigraphy among the studied sections, we present preliminary radioisotopic age determinations of, and correlations among, these tuffs using the zircon U-Pb system, 40Ar/39Ar dating where possible, as well as mineral chemistry. Our results include the first dated tuff in the Ochoan Series that lies within the Alibates Formation which underlies the Dewey Lake Fm. Other samples in progress from the various tuffs in the region, in combination with results from magneto- and chemostratigraphy, will significantly expand the areal coverage of these strata and lead towards a greatly improved chronostratigraphic framework.

U-Pb geochronologic constraints on the origin of a unique monazite- xenotime gneiss, Hudson Highlands, New York

USGS Publications Warehouse

Aleinikoff, J.N.; Grauch, R.I.

1990-01-01

A unique rock composed almost entirely of equal proportions of monazite and xenotime occurs as a small, lenticular body (2 ?? 0.5 ?? 0.3 m) in association with paragneiss, migmatite, and Canada Hill Granite in an outcrop in the Hudson Highlands of southeastern New York. The paragneiss contains detrital zircon (207Pb/206Pb ages of 1150-1460 Ma), monazite, and xenotime (both dated at about 1000 Ma). Zircons from the monazite-xenotime gneiss contain dark, rounded cores and clear rims, a morphology suggestive of derivation from the paragneiss, with subsequent metamorphic overgrowth. We conclude, based on results from xenotime and zircon rims, that the monazite-xenotime gneiss formed at about 985 Ma. Based on zircon morphology and age relations within the outcrop, we prefer a metasomatic origin over other possibilities such as a paleo-placer or anatectic restite. -from Authors

Crustal xenoliths in post-collisional Variscan lamprophyres: records of late Variscan collision and orogenic extension

NASA Astrophysics Data System (ADS)

Soder, Christian; Ludwig, Thomas; Schwarz, Winfried; Trieloff, Mario

2017-04-01

Crustal xenoliths entrained in post-collisional shoshonitic lamprophyres from the Variscan Odenwald (Mid-German Crystalline Zone, MGCZ) include felsic granulites (garnet, quartz, plagioclase, K-feldspar, biotite, omphacite, rutile) and basaltic eclogites (omphacite, garnet, quartz, kyanite, phengite, epidote, rutile). Classical thermobarometry, Zr-in-rutile thermometry and equilibrium phase diagrams reveal temperatures of 700-800°C and pressures of 1.7-1.8 GPa. Both lithologies record isothermal decompression resulting in partial melting at still elevated pressures (1.3-1.5 kbar) before entrainment into the magma. The development of diverse fine-grained microstructures is linked to the interaction with the rising melt. The eclogitic garnet preserves compositional sector zonation patterns, which indicate rapid crystal growth, shortly followed by overgrowth/recrystallization during decompression. The preservation of these zonation patterns indicates crystallization immediately before the lamprophyre magmatism. These findings are supported by SIMS U-Pb dating of zircon rims, which gave ages of 330±3 Ma for both lithologies, indistinguishable from the published age of lamprophyre emplacement. Therefore, the xenoliths are a unique document of the late Variscan collisional process with marked crustal thickening to 60 km and a subsequent decompression event. Magmatic protolith ages are 430 Ma for the basaltic eclogite and 2.1 Ga for the felsic granulite. Silurian magmatism is well established within the MGCZ while the Paleoproterozoic age represents a hitherto unknown magmatic event.

Age of UHP metamorphism in the Western Mediterranean: Insight from rutile and minute zircon inclusions in a diamond-bearing garnet megacryst (Edough Massif, NE Algeria)

NASA Astrophysics Data System (ADS)

Bruguier, Olivier; Bosch, Delphine; Caby, Renaud; Vitale-Brovarone, Alberto; Fernandez, Laure; Hammor, Dalila; Laouar, Rabah; Ouabadi, Aziouz; Abdallah, Nachida; Mechati, Mehdi

2017-09-01

Diamond-bearing UHP metamorphic rocks witness for subduction of lithospheric slabs into the mantle and their return to shallow levels. In this study we present U-Pb and trace elements analyses of zircon and rutile inclusions from a diamond-bearing garnet megacryst collected in a mélange unit exposed on the northern margin of Africa (Edough Massif, NE Algeria). Large rutile crystals (up to 300 μm in size) analyzed in situ provide a U-Pb age of 32.4 ± 3.3 Ma interpreted as dating the prograde to peak subduction stage of the mafic protolith. Trace element analyses of minute zircons (≤30 μm) indicate that they formed in equilibrium with the garnet megacryst at a temperature of 740-810 °C, most likely during HP retrograde metamorphism. U-Pb analyses provide a significantly younger age of 20.7 ± 2.3 Ma attributed to exhumation of the UHP units. This study allows bracketing the age of UHP metamorphism in the Western Mediterranean Orogen to the Oligocene/early Miocene, thus unambiguously relating UHP metamorphism to the Alpine history. Exhumation of these UHP units is coeval with the counterclockwise rotation of the Corsica-Sardinia block and most likely resulted from subduction rollback that was driven by slab pull.

Constraining the Flux of Impactors Postdating Heavy Bombardment Using U-Pb Ages of Impact Glasses

NASA Technical Reports Server (NTRS)

Nemchin, A. A.; Norman, M. L.; Ziegler, R. A.; Grange, M. L.

2013-01-01

Spherules of glass varying in size from a few micrometres to a few millimetres are common in the lunar regolith. While some of these glass beads are products of pyroclastic fire fountains others originate as impact melt ejected from the target that breaks into small droplets and solidifies as spherical particles while raining back to the lunar surface. These glasses preserve information about the chemical composition of the target and often contain sufficient amount of radioactive nuclides such as 40K to enable Ar-40-Ar-39 dating of individual beads. Studies measuring the age of glass beads have been used in attempts to establish variations in the flux of impactors hitting the Moon, particularly during the period that postdates the formation of major impact basins [1,2]. These studies proposed a possibility of spike in the impact flux about 800 Ma [2] and over the last 400 Ma [1]. More recently U-Th-Pb isotopic systems have been also utilized to determine the age of impact glasses from the Apollo 17 regolith [3]. Our aim is to extend the application of the U-Pb system in impact glasses to spherules isolated from Apollo 14 soil 14163 in an attempt to further investigate the applicability of this isotopic system to the chronology of impact glass beads and gain additional information on the impact flux in the inner Solar system.

Early Jurassic mafic dykes from the Aigao uranium ore deposit in South China: Geochronology, petrogenesis and relationship with uranium mineralization

NASA Astrophysics Data System (ADS)

Zhang, Di; Zhao, Kui-Dong; Chen, Wei; Jiang, Shao-Yong

2018-05-01

Mafic dykes are abundant and widely distributed in many granite-hosted uranium ore deposits in South China. However, their geochronology, petrogenesis and relationship with uranium mineralization were poorly constrained. In this study, apatite U-Pb dating, whole-rock major and trace element and Sr-Nd-Pb isotope analysis were conducted for the dolerite dykes from the Aigao uranium ore deposit. Apatite U-Pb isotopic data indicate that the mafic dykes were emplaced at Early Jurassic (189 ± 4 Ma), which provides new evidence for the rarely identified Early Jurassic magmatism in South China. Pyroxene from the dykes is mainly augite, and plagioclase belongs to albite. The dolerite samples have relatively low SiO2 contents (45.33-46.79 wt%), relatively high total alkali contents (K2O + Na2O = 4.11-4.58 wt%) and Al2O3 contents (13.39-13.80 wt%), and medium MgO contents (4.29-5.16 wt%). They are enriched in Nb, Ta, Ti, rare earth elements and depleted in Rb, K, Sr, Th, showing the typical OIB-like geochemical affinity. All the dolerite samples show homogeneous Sr-Nd-Pb isotopic compositions, with (87Sr/86Sr)i varying from 0.706049 to 0.707137, εNd(t) from +4.6 to +5.2, 206Pb/204Pb from 19.032 to 19.126 and 207Pb/204Pb from 15.641 to 15.653. The mafic dykes in the Aigao deposit should be derived from the partial melting of the asthenospheric mantle and formed in a within-plate extensional environment. The emplacement age of the mafic dykes is older than the uranium mineralization age. Therefore, CO2 in ore-forming fluids couldn't originate from the basaltic magma as suggested by previous studies. The dolerite dykes might only provide a favorable reducing environment to promote the precipitation of uraninite from oxidize hydrothermal fluids.

Linking sedimentary total organic carbon to 210Pbex chronology from Changshou Lake in the Three Gorges Reservoir Region, China.

PubMed

Anjum, Raheel; Gao, Jinzhang; Tang, Qiang; He, Xiubin; Zhang, Xinbao; Long, Yi; Shi, Zhonglin; Wang, Mingfeng

2017-05-01

The influences of total organic carbon (TOC) and total nitrogen (TN) on Lead-210 ( 210 Pb) dating have recently been of increasing concern in lacustrine research. Sediment core from Changshou Lake in the Longxi catchment was investigated for influence of TOC on 210 Pb dating. Lead-210 excess ( 210 Pb ex ), Cesium-137 ( 137 Cs) activities, TOC, TN, and particle size were measured. We proposed a dating index based on 137 Cs chronology and particle size distribution of the lake sediment profile and rainfall erosivities calculated from Longxi catchment metrological records. Increasing trends in TOC and TN were specifically caused by commercial cage fish farming after 1989. The statistically significant correlation between 210 Pb ex activity, TOC (0.61, p = 0.04) and TN (0.51, p = 0.04), respectively explained post-1989 210 Pb scavenging. The 210 Pb ex activity was closely related with coupled peaks of TOC and TN from mass depth 5-10 g cm -2 . Higher TOC/TN ratio (8.33) indicated submerged macrophytes and native aquatic algal growth as main source of carbon from enhanced primary productivity because of massive fertilizer use and coherent climate warming. The study supported key hypothesis on vital role of fertilizer usage and algal derived TOC in controlling sedimentary 210 Pb ex activity at Changshou Lake sediment. 137 Cs profile and erosive events as time markers provided reliable and consistent sedimentation rate of (1.6 cm y -1 ). 210 Pb ex activity decayed exponentially after peak at mass depth 5.68 g cm -2 . Therefore, violation of 210 Pb dating primary assumptions made it inappropriate for sediment dating at Changshou Lake. TOC content must be considered while using 210 Pb as dating tool for lake sediment profiles. Copyright © 2017 Elsevier Ltd. All rights reserved.

Zircons as a Probe of Early Luanr History

NASA Astrophysics Data System (ADS)

Crow, C. A.; McKeegan, K. D.; Gilmour, J. D.; Crowther, S. A.; Talor, D. J.

2013-09-01

Zircons are ideal for investigating the early lunar bombardment because we can measure both U-Pb crystallization ages and fissiongenic Xe degassing ages for the same crystal. We report U-Pb, Pb-Pb and U-Xe ages for three lunar zircons.

40Ar/39Ar dating and zircon chronochemistry for the Izu-Bonin rear arc, IODP site U1437

NASA Astrophysics Data System (ADS)

Schmitt, A. K.; Konrad, K.; Andrews, G. D.; Horie, K.; Brown, S. R.; Koppers, A. A. P.; Busby, C.; Tamura, Y.

2016-12-01

The scientific objective of IODP Expedition 350 drilling at Site U1437 (31°47.390'N, 139°01.580'E) was to reveal the "missing half of the subduction factory": the rear arc of a long-lived intraoceanic subduction zone. Site U1437 lies in a 50 km long and 20 km wide volcano-bounded basin, 90 km west of the Izu arc front, and is the only IODP site drilled in the rear arc. The Izu rear arc is dominated by Miocene basaltic to dacitic seamount chains, which strike at a high angle to the arc front. Radiometric dating targeted a single igneous unit (1390 mbsf), and fine to coarse volcaniclastic units for which we present zircon and 40Ar/39Ar (hornblende, plagioclase, and groundmass) age determinations. All zircons analyzed as grain separates were screened for contamination from drill-mud (Andrews et al., 2016) by analyzing trace elements and, where material was available, O and Hf isotope compositions. Igneous Unit 1 is a rhyolite sheet and yielded concordant in-situ and crystal separate U-Pb zircon ages (13.7±0.3 Ma; MSWD = 1.3; n = 40 spots), whereas the 40Ar/39Ar hornblende plateau age (12.9±0.3; MSWD = 1.1; n = 9 steps) is slightly younger, possibly reflecting pre-eruptive zircon crystallization, or alteration of hornblende. U-Pb zircon and 40Ar/39Ar plateau ages from samples above igneous Unit 1 are concordant with biostratigraphic and paleomagnetic ages (available to 1300 mbsf), but plagioclase and groundmass samples below 1300 m become younger with depth, hinting at post-depositional alteration. A single zircon from 1600 mbsf yielded a U-Pb age of 15.4±1.8 Ma; its trace element composition resembles other igneous zircons from U1437, and is tentatively interpreted as a Middle Miocene age for the lowermost lithostratigraphic unit VII. Oxygen and Hf isotopic values of igneous zircon indicate mantle origins, with some influence of assimilation of hydrothermally altered oceanic crust evident in sub-mantle oxygen isotopic compositions. Lessons from site U1437 are that integrated chronochemistry is essential for achieving accurate age models in oceanic drilling. Reference: Andrews, G. D., Schmitt, A. K., Busby, C. J., Brown, S. R., Blum, P., & Harvey, J. (2016). Age and compositional data of zircon from sepiolite drilling mud to identify contamination of ocean drilling samples. G3. doi: 10.1002/2016GC006397.

Exotic island arc Paleozoic terranes on the eastern margin of Gondwana: Geochemical whole rock and zircon U-Pb-Hf isotope evidence from Barry Station, New South Wales, Australia

NASA Astrophysics Data System (ADS)

Manton, Ryan J.; Buckman, Solomon; Nutman, Allen P.; Bennett, Vickie C.

2017-08-01

Early Paleozoic intra-oceanic terranes crop out along the Peel-Manning Fault System, in the southern New England Orogen, NSW Australia. These are the Cambrian ophiolitic Weraerai terrane and the Siluro-Devonian island arc Gamilaroi terrane. There has been debate whether these terranes formed at the Gondwana margin or if they are intra-oceanic, and were accreted to Gondwana later in the Paleozoic. Major-trace-REE elemental data indicate Weraerai terrane formed in a supra-subduction environment. Rare zircons extracted from Weraerai terrane gabbro-plagiogranite suites at Barry Station yield a U-Pb zircon date of 504.9 ± 3.5 Ma with initial εHf values of + 11.1 indicating a juvenile source. Amphibole-bearing felsic dykes and net-vein complexes are also found within the gabbro with a U-Pb zircon date of 503.2 ± 5.7 Ma and initial εHf values of + 11.6. These are coeval in age with their host rocks and we propose they represent partial melts of the mafic crust during the circulation of seawater. The Gamilaroi trondhjemites of prehnite-pumpellyite-greenschist metamorphic grade terrane yielded very few zircons with an age of 413 ± 8.7 Ma. Zircon initial εHf values range from + 5.0 to + 2.9, indicating an input from an evolved crustal source, unlike the purely oceanic Weraerai terrane. Gamilaroi terrane trondhjemites are enriched in LREE have low K2O and K2O/Na2O ratios and strong negative Nb anomalies consistent with supra-subduction zone environments. Multiple subduction zones may well have existed within the Panthalassa Ocean during the early-mid Paleozoic with the Weraerai-Gamilaroi being accreted onto the Gondwanan margin during the latest Devonian.

Late Cretaceous (ca. 95 Ma) magnesian andesites in the Biluoco area, southern Qiangtang subterrane, central Tibet: Petrogenetic and tectonic implications

NASA Astrophysics Data System (ADS)

He, Haiyang; Li, Yalin; Wang, Chengshan; Zhou, Aorigele; Qian, Xinyu; Zhang, Jiawei; Du, Lintao; Bi, Wenjun

2018-03-01

The tectonic evolutionary history of the Lhasa and Qiangtang collision zones remains hotly debated because of the lack of pivotal magmatic records in the southern Qiangtang subterrane, central Tibet. We present zircon U-Pb dating, whole-rock major and trace-element geochemical analyses, and Sr-Nd isotopic data for the newly discovered Biluoco volcanic rocks from the southern Qiangtang subterrane, central Tibet. Zircon U-Pb dating reveals that the Biluoco volcanic rocks were crystallized at ca. 95 Ma. The samples are characterized by low SiO2 (50.26-54.53 wt%), high Cr (109.7-125.92 ppm) and Ni (57.4-71.58 ppm), and a high Mg# value (39-56), which plot in the magnesian andesites field on the rock classification diagram. They display highly fractionated rare earth element patterns with light rare earth element enrichment ([La/Yb]N = 21.04-25.24), high Sr/Y (63.97-78.79) and no negative Eu anomalies (Eu/Eu* = 0.98-1.04). The Biluoco volcanic rocks are depleted in Nb, Ta and Ti and enriched in Ba, Th, U and Pb. Moreover, the eight samples of Biluoco volcanic rocks display constant (87Sr/86Sr)i ratios (0.70514-0.70527), a positive εNd(t) value (2.16-2.68) and younger Nd model ages (0.56-0.62 Ga). These geochemical signatures indicate that the Biluoco volcanic rocks were most likely derived from partial melting of the mantle wedge peridotite metasomatized by melts of subducted slab and sediment in the subducted slab, invoked by asthenospheric upwelling resulting from the slab break-off of the northward subduction of the Bangong-Nujiang oceanic lithosphere. Identification of ca. 95 Ma Biluoco magnesian andesites suggests they were a delayed response of slab break-off of the northward subduction of the Bangong-Nujiang oceanic lithosphere at ca. 100 Ma.

Triassic arc-derived detritus in the Triassic Karakaya accretionary complex was not derived from either the S Eurasian margin (Istanbul terrane) or the N Gondwana margin (Taurides)

NASA Astrophysics Data System (ADS)

Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair H. F.; Gerdes, Axel; Zulauf, Gernold

2014-05-01

We present new U-Pb zircon source age data for Upper Triassic sandstones of the Istanbul Terrane (S Eurasian margin) and also for Triassic sandstones of the Taurides (N Gondwana margin). The main aim is to detect and quantify the contribution of Triassic magmatism as detritus to either of these crustal blocks. This follows the recent discovery of a Triassic magmatic arc source for the Triassic sandstones of the Palaeotethyan Karakaya subduction-accretion complex (Ustaömer et al. 2013; this meeting). Carboniferous (Variscan) zircon grains also form a significant detrital population, plus several more minor populations. Six sandstone samples were studied, two from the İstanbul Terrane (Bakırlıkıran Formation of the Kocaeli Triassic Basin) and four from the Tauride Autochthon (latest Triassic Üzümdere Formation and Mid-Triassic Kasımlar Formations; Beyşehir region). Detrital zircon grains were dated by the laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) U-Pb method at Goethe University, Frankfurt. Our results do not reveal Triassic detritus in the Üzümdere Formation. The U-Pb age of the analysed zircon grains ranges from 267 Ma to 3.2 Ga. A small fraction of Palaeozoic zircons are Permian (267 to 296 Ma), whereas the remainder are Early Palaeozoic. Ordovician grains (4%) form two age clusters, one at ca. 450 Ma and the other at ca. 474 Ma. Cambrian-aged grains dominate the zircon population, while the second largest population is Ediacaran (576 to 642 Ma). Smaller populations occur at 909-997 Ma, 827-839 Ma, 1.8-2.0 Ga and 2.4-2.6 Ga. The sandstones of the Kasımlar Formation have similar zircon age cluster to those of the somewhat younger Üzümdere Formation, ranging from 239 Ma to 2.9 Ga. A few grains gave Anisian ages. Cambrian zircon grains are less pronounced than in the Kasımlar Formation compared to the Üzümdere Formation. The detrital zircon record of Tauride sandstones, therefore, not indicates significant contribution of Triassic or Carboniferous (Variscan) arc sources, in marked contrast to those of the Triassic Karakaya subduction complex. In comparison, the ages of the analysed zircons in the Upper Triassic sandstones of the Istanbul Terrane range from 294 Ma to 3.1 Ga. Triassic zircons are again absent, while Variscan-aged zircons (294 to 339 Ma) dominate the zircon population. Additional zircon populations are dated at 554 to 655 Ma, 0.9 to 1.2 Ga, 1.5 Ga, 1.65 Ga, 2.0 to 2.15 and 2.5 to 2.8 Ga. The Precambrian zircon age spectra are compatible with derivation from an Avalonian/Amazonian/Baltic crustal provenance. In summary, there is no evidence in either the Triassic sandstones of the İstanbul Terrane or of the Taurides of the Triassic magmatic arc source that dominates the Triassic Karakaya subduction-accretion complex. Where then was the source of the Karakaya arc detritus? A likely option is that the Karakaya subduction-accretion complex is an exotic terrane that was detached from a source magmatic arc and displaced to its present location, probably prior the initial deposition of the Early Jurassic cover sediments. This study was supported by TUBITAK, Project No: 111R015

U-Pb Geochronology of Hydrous Silica (Siebengebirge, Germany)

NASA Astrophysics Data System (ADS)

Tomaschek, Frank; Nemchin, Alexander; Geisler, Thorsten; Heuser, Alexander; Merle, Renaud

2015-04-01

Low-temperature, hydrous weathering eventually leads to characteristic products such as silica indurations. Elevated U concentrations and the ability of silica to maintain a closed system permits silica to be dated by the U-Pb method, which, in turn, will potentially allow constraining the timing of near-surface processes. To test the feasibility of silica U-Pb geochronology, we sampled opal and chalcedony from the Siebengebirge, Germany. This study area is situated at the terminus of the Cenozoic Lower Rhine Basin on the Rhenish Massif. The investigated samples include silicified gravels from the Mittelbachtal locality, renowned for the embedded wood opal. Structural characterization of the silica phases (Raman spectroscopy) was combined with in situ isotopic analyses, using ion microprobe and LA-ICPMS techniques. In the Siebengebirge area fluviatile sediments of Upper Oligocene age were covered by an extended trachyte tuff at around 25 Ma. Silica is known to indurate some domains within the tuff and, in particular, certain horizons within the subjacent fluviatile sediments ('Tertiärquarzite'). Cementation of the gravels occurred during at least three successive growth stages: early paracrystalline silica (opal-CT), fibrous chalcedony, and late microcrystalline quartz. It has traditionally been assumed that this silica induration reflects intense weathering, more or less synchronous with the deposition of the volcanic ashes. Results from U-Pb geochronology returned a range of discrete 206Pb-238U ages, recording a protracted silicification history. For instance, we obtained 22 ± 1 Ma for opal-CT cement from a silicified tuff, 16.6 ± 0.5 Ma for silicified wood and opal-CT cement in the fluviatile gravels, as well as 11 ± 1 Ma for texturally late chalcedony. While silicification of the sampled tuff might be contemporaneous with late-stage basalts, opaline silicification of the subjacent sediments and their wood in the Mittelbachtal clearly postdates active Siebengebirge volcanism, and the clastic sedimentation by about 8 Myr. To account for the age discrepancies, opal-CT formation might be a local and episodic phenomenon, reflecting progressive denudation of the trachyte tuff cover. Alternatively, the dominant silicification event of the Mittelbachtal silcretes could be of regional significance (Middle Miocene Climatic Optimum). Our relatively fast approach by LA-ICPMS analysis will be used to further expand the database.

Zircons as a Probe of Early Lunar Impact History

NASA Astrophysics Data System (ADS)

Crow, C. A.; McKeegan, K. D.; Gilmour, J. D.; Crowther, S. A.; Taylor, D. J.

2013-08-01

Zircons are ideal for investigating the early lunar bombardment because we can measure both U-Pb crystallization ages and fissiongenic Xe degassing ages for the same crystal. We report U-Pb, Pb-Pb and U-Xe ages for two lunar zircons.

Solving the Martian meteorite age conundrum using micro-baddeleyite and launch-generated zircon.

PubMed

Moser, D E; Chamberlain, K R; Tait, K T; Schmitt, A K; Darling, J R; Barker, I R; Hyde, B C

2013-07-25

Invaluable records of planetary dynamics and evolution can be recovered from the geochemical systematics of single meteorites. However, the interpreted ages of the ejected igneous crust of Mars differ by up to four billion years, a conundrum due in part to the difficulty of using geochemistry alone to distinguish between the ages of formation and the ages of the impact events that launched debris towards Earth. Here we solve the conundrum by combining in situ electron-beam nanostructural analyses and U-Pb (uranium-lead) isotopic measurements of the resistant micromineral baddeleyite (ZrO2) and host igneous minerals in the highly shock-metamorphosed shergottite Northwest Africa 5298 (ref. 8), which is a basaltic Martian meteorite. We establish that the micro-baddeleyite grains pre-date the launch event because they are shocked, cogenetic with host igneous minerals, and preserve primary igneous growth zoning. The grains least affected by shock disturbance, and which are rich in radiogenic Pb, date the basalt crystallization near the Martian surface to 187 ± 33 million years before present. Primitive, non-radiogenic Pb isotope compositions of the host minerals, common to most shergottites, do not help us to date the meteorite, instead indicating a magma source region that was fractionated more than four billion years ago to form a persistent reservoir so far unique to Mars. Local impact melting during ejection from Mars less than 22 ± 2 million years ago caused the growth of unshocked, launch-generated zircon and the partial disturbance of baddeleyite dates. We can thus confirm the presence of ancient, non-convecting mantle beneath young volcanic Mars, place an upper bound on the interplanetary travel time of the ejected Martian crust, and validate a new approach to the geochronology of the inner Solar System.

Radiochronometry in the CMX-4 Exercise-Draft

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kristo, M. J.; Williams, R.; Gaffney, A. M.

In a recent international exercise, 10 international nuclear forensics laboratories successfully performed radiochronometry on 3 low-enriched uranium oxide samples, providing 12 analytical results using 3 different parent-daughter pairs serving as independent chronometers. The vast majority of the results were consistent with one another and consistent with the known processing history of the materials. In general, for these particular samples, mass spectrometry gave more accurate and more precise analytical results than decay counting measurements. In addition, the concordance of the 235U- 231Pa and 234U- 230Th chronometers confirmed the validity of the age dating assumption, increasing our confidence in the resulting conclusions.

Recovery of NORM from scales generated by oil extraction.

PubMed

Al Attar, Lina; Safia, Bassam; Ghani, Basem Abdul; Al Abdulah, Jamal

2016-03-01

Scales, containing naturally occurring radioactive materials (NORM), are a major problem in oil production that lead to costly remediation and disposal programmes. In view of environmental protection, radio and chemical characterisation is an essential step prior to waste treatment. This study focuses on developing of a protocol to recover (226)Ra and (210)Pb from scales produced by petroleum industry. X-ray diffractograms of the scales indicated the presence of barite-strontium (Ba0.75Sr0.25SO4) and hokutolite (Ba0.69Pb0.31SO4) as main minerals. Quartz, galena and Ca2Al2SiO6(OH)2 or sphalerite and iron oxide were found in minor quantities. Incineration to 600 °C followed by enclosed-digestion and acid-treatment gave complete digestion. Using (133)Ba and (210)Pb tracers as internal standards gave recovery ranged 87-91% for (226)Ra and ca. 100% for (210)Pb. Radium was finally dissolved in concentrated sulphuric acid, while (210)Pb dissolved in the former solution as well as in 8 M nitric acid. Dissolving the scales would provide better estimation of their radionuclides contents, facilitate the determination of their chemical composition, and make it possible to recycle NORM wastes in terms of radionuclides production. Copyright © 2015 Elsevier Ltd. All rights reserved.

Distinct 238U/235U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

NASA Astrophysics Data System (ADS)

Tissot, François L. H.; Dauphas, Nicolas; Grove, Timothy L.

2017-09-01

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar System formation, when several short-lived nuclides (e.g., 26Al-26Mg, 53Mn-53Cr, 182Hf-182W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an ;assumed; constant 238U/235U ratio (i.e., Pb-Pb ages published before 2009-2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D'Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from -0.17 to -1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (53Mn/55Mn)0 ratio in the Early Solar System (ESS) is recalculated as being (7 ± 1) × 10-6 at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions - CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs. A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ238U values are consistent with fractionation during magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction.

Zirconology of lherzolites in the Nurali Massif

NASA Astrophysics Data System (ADS)

Krasnobaev, A. A.; Rusin, A. I.; Anfilogov, V. N.; Valizer, P. M.; Busharina, S. V.; Medvedeva, E. V.

2017-06-01

The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445-448 and 439-440 Ma) wass the time of lherzolite magmatism of 10-15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro-diorite intrusions crowning in the Nurali Massif.

Uranium metallogenesis of the peraluminous leucogranite from the Pontivy-Rostrenen magmatic complex (French Armorican Variscan belt): the result of long-term oxidized hydrothermal alteration during strike-slip deformation

NASA Astrophysics Data System (ADS)

Ballouard, C.; Poujol, M.; Mercadier, J.; Deloule, E.; Boulvais, P.; Baele, J. M.; Cuney, M.; Cathelineau, M.

2018-06-01

In the French Armorican Variscan belt, most of the economically significant hydrothermal U deposits are spatially associated with peraluminous leucogranites emplaced along the south Armorican shear zone (SASZ), a dextral lithospheric scale wrench fault that recorded ductile deformation from ca. 315 to 300 Ma. In the Pontivy-Rostrenen complex, a composite intrusion, the U mineralization is spatially associated with brittle structures related to deformation along the SASZ. In contrast to monzogranite and quartz monzodiorite (3 < U < 9 ppm; Th/U > 3), the leucogranite samples are characterized by highly variable U contents ( 3 to 27 ppm) and Th/U ratios ( 0.1 to 5) suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and/or surface weathering. U-Pb dating of uranium oxides from the deposits reveals that they mostly formed between ca. 300 and 270 Ma. In monzogranite and quartz monzodiorite, apatite grains display magmatic textures and provide U-Pb ages of ca. 315 Ma reflecting the time of emplacement of the intrusions. In contrast, apatite grains from the leucogranite display textural, geochemical, and geochronological evidences for interaction with U-rich oxidized hydrothermal fluids contemporaneously with U mineralizing events. From 300 to 270 Ma, infiltration of surface-derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed U deposits. This phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with U-rich granitic bodies.

Single-crystal 40Ar/39Ar incremental heating reveals bimodal sanidine ages in the Bishop Tuff

NASA Astrophysics Data System (ADS)

Andersen, N. L.; Jicha, B. R.; Singer, B. S.

2015-12-01

The 650 km3 Bishop Tuff (BT) is among the most studied volcanic deposits because it is an extensive marker bed deposited just after the Matuyama-Brunhes boundary. Reconstructions of the vast BT magma reservoir from which high-silica rhyolite erupted have long influenced thinking about how large silicic magma systems are assembled, crystallized, and mixed. Yet, the longevity of the high silica rhyolitic melt and exact timing of the eruption remain controversial due to recent conflicting 40Ar/39Ar sanidine vs. SIMS and ID-TIMS U-Pb zircon dates. We have undertaken 21 40Ar/39Ar incremental heating ages on 2 mm BT sanidine crystals from pumice in 3 widely separated outcrops of early-erupted fall and flow units. Plateau ages yield a bimodal distribution: a younger group has a mean of 766 ka and an older group gives a range between 772 and 782 ka. The younger population is concordant with the youngest ID-TIMS and SIMS U-Pb zircon ages recently published, as well as the astronomical age of BT in marine sediment. Of 21 crystals, 17 yield older, non-plateau, steps likely affected by excess Ar that would bias traditional 40Ar/39Ar total crystal fusion ages. The small spread in older sanidine ages, together with 25+ kyr of pre-eruptive zircon growth, suggest that the older sanidines are not partially outgassed xenocrysts. A bimodal 40Ar/39Ar age distribution implies that some fraction of rhyolitic melt cooled below the Ar closure temperature at least 10 ky prior to eruption. We propose that rapid "thawing" of a crystalline mush layer released older crystals into rhyolitic melt from which sanidine also nucleated and grew immediately prior to the eruption. High precision 40Ar/39Ar dating can thus provide essential information on thermo-physical processes at the millenial time scale that are critical to interpreting U-Pb zircon age distributions that are complicated by large uncertainties associated with zircon-melt U-Th systematics.

The Younger Age Limit of Metasedimentary Protolith Formation of the Lower Part of the Udokan Group Rocks (Aldan Shield)

NASA Astrophysics Data System (ADS)

Kotov, A. B.; Salnikova, E. B.; Kovach, V. P.; Velikoslavinskii, S. D.; Sklyarov, E. V.; Gladkochub, D. P.; Larin, A. M.; Tolmacheva, E. V.; Fedoseenko, A. M.; Plotkina, Yu. V.

2018-04-01

Biotite plagiogranite intruding sediments of the Kodar Sub-Group of the Udokan Group that have both undergone amphibolite grade alterations has been dated by the U-Pb ID TIMS technique using zircon to 2105 ± 6 Ma. This age estimate to a first approximation corresponds to the younger age limit of deposition of the siliciclastics in the lower section of the Udokan Group.

U-Pb Geochronology and Hf-isotope constrains on Formation of Archaean Crust From the Lewisian of NW Scotland, Great Britain

NASA Astrophysics Data System (ADS)

Crowley, Q. G.; Noble, S. R.; Key, R.

2006-12-01

The Lewisian complex of NW Scotland is dominantly composed of Archaean tonalitic to granodioritic gneisses, ultramafic bodies and minor metasedimentary components. Although the area is internationally well known and has been much studied for over a century, the precise timing of crustal forming events has proven difficult to ascertain. We present data from both in-situ laser ablation (LA) ICP-MS and an adaptation of a new U-Pb chemical abrasion ID-TIMS technique (Mattinson 2006) applied to multi-age component zircons from the Assynt block of this region. The new data reveal a previously unrecognised complexity and provide the first unequivocal proof of an Archean metamorphic event in the area. In a wider context the data also elucidate some of the processes involved in early global crust formation and plate tectonic events. In-situ LA-ICPMS U-Pb dating has indicated a ca 2.8Ga protolith age for a tonalite gneiss with evidence for a ca. 3.6Ga xenocrystic component (the oldest discovered in the UK). Non-conventional U-Pb ID-TIMS utilising a combination of high-temperature annealing followed by multi-step incremental dissolution on single grains has dated zircon growth at ca 2.7Ga (Badcallian) and 2.5Ga (Inverian) with later Pb-loss occurring at ca 1.9Ga and ca 1.7Ga (early and late Laxfordian respectively). This latter method combines a pseudo-spatial resolution normally associated with an in-situ technique but benefits from the high-precision analysis of ID-TIMS. Zircon Hf isotopes indicate that some rocks from the Assynt area are typical of Archaean continental crust (epsilon Hf ca -1. The tonalite gneisses however have strongly negative epsilon Hf values of -7 to -10 indicating a more complex history of derivation through partial melting of ancient crust with residual garnet as a long- lived control on Hf. Archaean events at ca. 3.6Ga, ca 2.8Ga and ca 2.7Ga have also been recorded in west Greenland (e.g. Mojzsis & Harrison 1999, Richards and Appel, 1987, Whitehouse et al 1999). This points towards widespread and important crustal forming events. Locally in Assynt in the Lewisian of NW Scotland these events are recorded with zircon crystallisation from a magma at ca 3.6Ga, partial melting and crustal recycling producing the tonalite gneiss protoliths at ca 2.8Ga, a prolonged lower crustal residence in granulite P-T conditions by ca 2.7Ga, further metamorphism in amphibolite conditions at ca 2.5Ga and later deformation associated with punctuated terrane amalgamation events between ca 1.9Ga and ca 1.7Ga.

EARTHTIME: Teaching geochronology to high school students

NASA Astrophysics Data System (ADS)

Bookhagen, Britta; Buchwaldt, Robert; McLean, Noah; Rioux, Matthew; Bowring, Samuel

2010-05-01

The authors taught an educational module developed as part of the EARTHTIME (www.earth-time.org) outreach initiative to 215 high school students from a Massachusetts (USA) High School as part of an "out-of-school" field trip. The workshop focuses on uranium-lead (U-Pb) dating of zircons and its application to solving a geological problem. The theme of our 2.5-hour module is the timing of the K-T boundary and a discussion of how geochronology can be used to evaluate the two main hypotheses for the cause of the concurrent extinction—the Chicxlub impact and the massive eruption of the Deccan Traps. Activities are divided into three parts: In the first part, the instructors lead hands-on activities demonstrating how rock samples are processed to isolate minerals by their physical properties. Students use different techniques, such as magnetic separation, density separation using non-toxic heavy liquids, and mineral identification with a microscope. We cover all the steps from sampling an outcrop to determining a final age. Students also discuss geologic features relevant to the K-T boundary problem and get the chance to examine basalts, impact melts and meteorites. In the second part, we use a curriculum developed for and available on the EARTHTIME website (http://www.earth-time.org/Lesson_Plan.pdf). The curriculum teaches the science behind uranium-lead dating using tables, graphs, and a geochronology kit. In this module, the students start by exploring the concepts of half-life and exponential decay and graphically solving the isotopic decay equation. Manipulating groups of double-sided chips labeled with U and Pb isotopes reinforces the concept that an age determination depends on the Pb/U ratio, not the absolute number of atoms present. Next, the technique's accuracy despite loss of parent and daughter atoms during analysis, as well as the use of isotopic ratios rather than absolute abundances, is explained with an activity on isotope dilution. Here the students determine the number of beads in a large bucket without counting them all by adding a precisely known number of "tracer" beads and averaging ratios from several small samples of the mixture. The (pre-counted) unknown quantity of beads represents the isotopic composition of zircon from four samples—the Deccan Trap basalts, the Chicxulub impact melt, and ash layers above and below the K-T boundary —and the students' measurements are used in the final part of the module. An introduction to statistical inference from small samples can also be added to this exercise. After this, the chemistry and physics behind geochemical laboratory techniques, ion exchange chromatography and isotope ratio measurements using a mass spectrometer, are explained using models, movies, posters, and analogies to familiar physics. In the final part, students engage in a summary exercise where they apply what they have learned to test the two competing hypotheses. Using the dates they calculated with isotope dilution and a graphical solution to the decay equation, they determine if the Chicxulub impact or the Deccan Trap volcanic eruption can explain the K/T boundary mass extinction. They learn the importance of measurement uncertainty in interpreting data and brainstorm how best to resolve this outstanding scientific problem. Feedback from written evaluations shows that teachers valued the interdisciplinary association of concepts from physics, chemistry and mathematics. Students enjoyed the hands-on exercises that gave them the opportunity to see how rocks can be broken down into mineral separates and individual zircons selected for analysis. The K/T-boundary exercise at the end was appreciated because it demonstrates an exciting application of geochronological methods to popular science.

U-Th-Pb systematics in three Apollo 14 basalts and the problem of initial Pb in lunar rocks.

NASA Technical Reports Server (NTRS)

Tera, F.; Wasserburg, G. J.

1972-01-01

The isotopic composition of Pb and the elemental concentration of U, Th and Pb were measured on 'total' rock samples 14053, 14073 and 14310 and on mineral separates of 14310 and 14053. These are the first Pb-U isochrons obtained for lunar basalts and indicate a reasonable solution to the previous discrepancy between the different methods of 'absolute' age determination. The resulting U-Pb isochron ages are compatible with the Rb-Sr and K-Ar ages on the same rocks. However, it is not possible to establish a precise time of 'crystallization' from the Pb-U data because of the small angle of intersection between the linear arrays and the concordia curve. These data show that total rock model ages do not in general yield crystallization ages. The data on 14310 and 14053 show that these rocks were formed containing a highly radiogenic initial lead which accounts for the excessively high total rock model ages by the U-Th-Pb method. The data prove that at the time of extrusion of some basalts, unsupported lead with extremely high Pb-207/Pb-206 ratios was added to the lunar surface.

Modeling Peat Ages Using 7Be Data to Account for Downwash of 210Pb

NASA Astrophysics Data System (ADS)

Manies, K.; Fuller, C.; Jones, M.

2016-12-01

In order to determine the amount of peat, and thus carbon, which has accumulated since the last thaw event, we are interested in dating the surface layers of boreal thermokarst bogs. However, there can often be a mismatch by several decades between dates obtained using 210Pb, 14C, or 137Cs. We found that 210Pb-based dates were almost always younger than 14C-based dates. One of the limitations often cited regarding the use of 210Pb dating for peatlands is the potential for this radionuclide to be transported down the soil profile, biasing the mean accumulation rate (MAR) towards higher values which, in turn, results in younger ages at a specific horizon. 7Be, which has similar depositional behaviors as 210Pb but a much shorter half-life (53.22 days), can be used to help determine if there is movement of 210Pb through surface layers and the depths to which 210Pb-bearing particles are transported (over the mean life of 7Be). These data can then be used in new models, such as the Linked Radionuclide aCcumulation model (LRC; Landis et al., 2016, http://dx.doi.org/10.1016/ j.gca.2016.02.2013), which account for 210Pb downwash when calculating soil horizon ages. To this end, we measured 7Be within a bog four times over the growing season. 7Be was found to 4 cm in May, reached its maximum depth of penetration in July (7 cm), and then receded again to 4 cm. The maximum integrated 7Be activity was also found in July. This pattern is similar to other studies which found 7Be deposition decreased over the rainy season. Next, we will calculate peat ages with models that include downwash of 210Pb, the depths of which will be based on the penetration depth of 7Be. These ages will be compared to 210Pb ages obtained with both the Constant Rate of Supply (CRS) and Constant Flux - Constant Sedimentation (CF:CS) models and to 137Cs- and 14C-derived ages. We anticipate that dates based on models that include some transport of 210Pb into the soil profile will provide more accurate peat formation dates and allow for more accurate carbon accumulation rates.

U-Th-Pb age of the Barwell chondrite - Anatomy of a 'discordant' meteorite

NASA Technical Reports Server (NTRS)

Unruh, D. M.; Tatsumoto, M.; Hutchison, R.

1979-01-01

A Pb-Pb internal isochron for the Barwell L5-6 chondrite yields an age of 4.530 plus or minus 0.005 billion years, using the measured U-238/U-235 ratio of 135.24 plus or minus .17. If the terrestrial U isotope composition is used, an age of 4.559 billion years is obtained. The Pb isotopic composition is distinctly different from that of a terrestrial contaminant found in the fusion crust of the Barwell stone. When the U-Th-Pb data are plotted on the concordia diagram, the data define a line that intersects the concordia curve at approximately 4.53 and 0 billion years, and nearly all of the data plot above the concordia curve, regardless of the initial Pb correction. This discordancy and the Pb isotopic composition of the triolite are attributed to a recent reequilibration of Pb and not to terrestrial contamination.

Petrogenesis of Late Jurassic granodiorites from Gutian, Fujian Province, South China: Implications for multiple magma sources and origin of porphyry Cu-Mo mineralization

NASA Astrophysics Data System (ADS)

Li, Bin; Jiang, Shao-Yong; Lu, An-Huai; Lai, Jian-Qing; Zhao, Kui-Dong; Yang, Tao

2016-11-01

The Gutian porphyry Cu-Mo deposit is a newly proved porphyry copper deposit in the coastal South China associated with granodioritic porphyries. In this study, zircon U-Pb ages and Hf isotope data, as well as geochemical and Sr-Nd-Pb-Re-Os isotopic compositions, are reported for these intrusions and minerals. Both zircon U-Pb and molybdenite Re-Os dating suggest that the Gutian granodiorite porphyries and related mineralization formed at 160 Ma. The Gutian granodiorites show a low-Mg adakitic geochemical affinity, with relatively high K2O but low Cr and Ni contents. These rocks have initial (87Sr/86Sr)i ratios of 0.7085 to 0.7097, negative εNd(t) values (- 12.5 to - 7.8), (206Pb/204Pb)t ratios of 18.048 to 18.241, (207Pb/204Pb)t ratios of 15.609 to 15.628, and (208Pb/204Pb)t ratios of 38.494 to 38.667. Zircons from the granodiorites have negative εHf(t) values of - 15.7 to - 8.5, which are close to those of Cathaysia crust-derived melts. Geochemical and Sr-Nd-Pb-Hf isotopic compositions suggest that they may be derived from Late Jurassic thickened juvenile lower crust. These lower crustal magma sources may not only contain pre-Proterozoic basement rocks, but also involve Triassic and Middle-Late Jurassic arc magmas within the lower crust, which were likely derived from an enriched mantle source associated with paleo-Pacific Plate subduction from the Middle to Late Jurassic. The Gutian ore-related granodiorites represent a new example for significant contributions of ancient subduction melts and enriched mantle-derived sources for porphyry-type magmatism and Cu-Mo mineralization, which occurred in response to an arc regime during the Middle to Late Jurassic in South China. Supplemental Table S2. Hf isotopic compositions of zircons from the studied rocks from the Gutian porphyry deposit in South China. Supplemental Table S3. Statistics for zircon U-Pb ages and Hf isotope compositions from Gutian granodiorites in South China Supplemental Table S4. Major element (wt.%) and trace element (ppm) concentrations of Gutian intrusions in Fujian Province, South China. Supplemental Table S5. Sr and Nd isotopic compositions of the studied rocks from the Gutian porphyry deposit in Fujian Province, South China. Supplemental Table S6. Pb isotopic compositions of the studied rocks from the Gutian porphyry deposit in South China. Supplemental Table S7. Re-Os isotopic compositions of molybdenite from the Gutian porphyry deposit in South China.

Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and delaminated newly underplated lower crust

NASA Astrophysics Data System (ADS)

Wang, Hao; Xu, Zhaowen; Lu, Xiancai; Fu, Bin; Lu, Jianjun; Yang, Xiaonan; Zhao, Zengxia

2016-01-01

The origin and tectonic setting of Early Cretaceous adakitic rocks from the Luxi terrane in the eastern North China Block (NCB) remain debated. To resolve this issue, we determined whole-rock geochemistry, zircon U-Pb ages, and in situ Hf-O isotopes of the Mengyin and Liujing adakitic porphyries from the Luxi terrane. Zircon U-Pb dating results reveal that both the Mengyin and Liujing plutons were emplaced during the Early Cretaceous, with weighted mean 206Pb/238U ages of 130 ± 1 Ma (2σ) and 131 ± 2 Ma (2σ), respectively. In addition, abundant Neoarchean-Paleoproterozoic inherited zircon cores are identified in the Mengyin adakitic porphyry with 207Pb/206Pb ages ranging from 2.53 to 2.42 Ga. Rocks of both plutons are silicic (SiO2 = 65.4-70.2 wt.%), metaluminous, and alkaline in composition, comprising mainly quartz syenite porphyries. Samples from both plutons are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr, and Ba), and light rare earth elements (LREEs), depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, and Ti), and heavy rare earth elements (HREEs), and have either positive or no Eu anomalies. In addition, both adakitic porphyries have high Mg# values (51-64), high Sr and La contents, low Y and Yb contents, and high Sr/Y (Mengyin = 149-264; Liujing = 58-110) and (La/Yb)N (Mengyin = 32.4-45.3; Liujing = 43.8-53.1) ratios, similar to adakitic rocks worldwide. The Mengyin adakitic porphyry has higher whole-rock εNd(t) values (-5.8 to - 4.1), more radiogenic Pb [(206Pb/204Pb)i = 18.35-18.39, (207Pb/204Pb)i = 15.55-15.56, (208Pb/204Pb)i = 38.20-38.23], higher zircon rim εHf(t) values (+ 3.3 to + 8.8) and δ18O values (+ 6.5‰ to + 7.9‰), and lower (87Sr/86Sr)i ratios (0.7049-0.7050) than the Liujing adakitic porphyry [εNd(t) = - 12.4 to - 12.2, (206Pb/204Pb)i = 17.63-17.72, (207Pb/204Pb)i = 15.56-15.58, (208Pb/204Pb)i = 37.76-37.94, εHf(t) = - 14.8 to - 11.2, δ18O = + 5.9‰ to + 7.1‰, (87Sr/86Sr)i = 0.7090-0.7091]. The Mengyin adakitic porphyry was most likely derived from partial melting of subducted oceanic slab with some input of NCB Neoarchean-Paleoproterozoic lower crust components. The Liujing adakitic porphyry was probably derived from partial melting of delaminated newly underplated thick lower crust, which then interacted with above asthenospheric mantle peridotite. Slab rollback together with the ridge subduction of the Paleo-Pacific slab was the most likely geodynamic mechanism for formation of the Early Cretaceous Mengyin and Liujing adakitic porphyries.

Anorogenic nature of magmatism in the Northern Baikal volcanic belt: Evidence from geochemical, geochronological (U-Pb), and isotopic (Pb, Nd) data

USGS Publications Warehouse

Neymark, L.A.; Larin, A.M.; Nemchin, A.A.; Ovchinnikova, G.V.; Rytsk, E. Yu

1998-01-01

The Northern Baikal volcanic belt has an age of 1.82-1.87 Ga and extends along the boundary between the Siberian Platform and the Baikal foldbelt. The volcanic belt is composed of volcanics of the Akitkan Group and granitic rocks of the Irel and Primorsk complexes. The geochemistry of the rocks points to the intraplate anorogenic nature of the belt. U-Pb zircon dating of the Chuya granitoids revealed that they are older (2020-2060 Ma) than the Northern Baikal volcanic belt and, thus, cannot be regarded as its component. Data on the Pb isotopic system of feldspars from the granitoids confirm the contemporaneity of all volcanic rocks of the belt except the volcanics of the upper portion of the Akitkan Group (Chaya Formation). Our data suggest its possibly younger (???1.3 Ga) age. The isotopic Nd and Pb compositions of the acid volcanic rocks provide evidence of the heterogeneity of their crustal protoliths. The volcanics of the Malaya Kosa Formation have ??Nd(T) = -6.1, ??2 = 9.36, and were most probably produced with the participation of the U-depleted lower continental crust of Archean age. Other rocks of the complex show ??Nd(T) from -0.1 to -2.4, ??2 = 9.78, and could have been formed by the recycling of the juvenile crust. The depletion of the Malaya Kosa volcanics in most LILEs and HFSEs compared with other acid igneous rocks of the belt possibly reflects compositional differences between the Late Archean and Early Proterozoic crustal sources. The basaltic rocks of the Malaya Kosa Formation (??Nd varies from -4.6 to -5.4) were produced by either the melting of the enriched lithospheric mantle or the contamination of derivatives of the depleted mantle by Early Archean lower crustal rocks, which are not exposed within the area. Copyright ?? 1998 by MAEe Cyrillic signK Hay??a/Interperiodica Publishing.

Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U-Pb zircon dating and Hf isotopic composition

NASA Astrophysics Data System (ADS)

Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Pan, Fa-Bin; Xu, Wang-Chun

2011-11-01

This integrated study of whole rock geochemistry, zircon U-Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U-Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25 Ma. The Jurassic granitic gneiss (165 Ma) exhibits εHf(t) values of + 1.4 to + 3.5. The late Cretaceous granite (81 Ma) shows variable εHf(t) values from - 0.9 to + 6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61 Ma) has εHf(t) values of + 5.4 to + 8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25 Ma) shows adakitic geochemical characteristics, with Sr/Y = 49.3-56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene-Eocene magmatism, metamorphism and anatexis are interpreted to result from roll-back and break-off of the subducted Neo-Tethyan slab that occurred in the early stage of the India-Asian collision, respectively. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at ~ 25 Ma.

Controlling factors of spatial and temporal preservation of the geochronological signal in sediments during an orogenic cycle

NASA Astrophysics Data System (ADS)

Rat, Juliette; Mouthereau, Frédéric; Bernet, Matthias; Brichau, Stéphanie; Balvay, Mélanie; Garzanti, Eduardo; Ando, Sergio

2017-04-01

Detrital content of sediments preserved in basins provide constraints on the nature of source rocks, dynamics of sediment transport, and potentially on tectonics and climate changes. U-Pb dating method on detrital zircon is ideally suited for provenance studies due to the ability of U-Pb age data to resist several orogenic cycles. However, with the aim to track sediment source evolution over a single orogenic cycle and determine characteristic time and parameters controlling the geochronological signal preservation throughout the cycle from rifting, mountain building to post-collision evolution, low-temperature thermochronology combined with sediment petrography are more appropriate than the U-Pb dating approach taken alone. To better understanding processes at play in the long-term geochronological signal preservation we focus on the sediment record associated with the Iberia plate tectonic evolution, which is part of the OROGEN research project, co-financed by BRGM, TOTAL & CNRS. The Iberian plate recorded a period of extension in the Late Jurassic, followed during the Early Cretaceous (Aptian-Albian) by a major thinning event documented by thick syn-rift sediments in intraplate basins and plate-scale heating/cooling of the Iberia crust, as argued by published fission track ages. Paleogeographic reconstructions that are based on stratigraphic and lithofacies analyses in northern Iberia (Iberian Range, Pyrenees and Basque-Cantabrians Range), describe a large domain of continental/fluvial and shallow-marine siliciclastic deposition. The related detrital content was then recycled during the subsequent Pyrenean orogenic phase in the Ebro foreland basin, and eventually transfer to the Mediterranean realm during post-orogenic re-excavation of the Ebro basin. In this study, we complete the published time-temperature paths in the mesozoic syn-rift basins by providing new thermo-chronological analyses of well-dated syn-collision and post-collision stratigraphic sections of the Ebro basin to determine thermal control on preservation through burial and geothermal evolution. We combined this study with sediments petrography analyses to identify relative control of source petrography, hydraulic sorting, alteration and diagenesis processes on the signal preservation during sediment transfer. All these observations will ultimately be incorporated in a geodynamic reconstruction of Iberia, and compared with age predictions from a model coupling surface processes and thermal evolution.

Neoarchean Subduction Recorded in the Northern Margin of the Yangtze Craton, South China

NASA Astrophysics Data System (ADS)

Zhang, S. B.; Zheng, Y. F.

2016-12-01

The Neoarchean is an important era during which plate tectonics began to operate widely on the earth and the continental crust compositions changed dramatically. However, reliable record of plate subduction has never been reported yet in the Yangtze Craton. Here we report geochemical studies on gneissic tonalite, trondhjemite and amphibolite in the Yudongzi Complex in the northern margin of the Yangtze Craton, which suggests that there is a plate subduction recorded in this area at about 2.7 Ga.The rocks in the Yudongzi Complex are gneissic granite, gneissic tonalite, amphibolite gneiss and magnetite quartzite. Most rocks are enriched in sodic. The gneissic granites show positive Eu anomalies, high (La/Yb)cn and Sr/Y ratios, low Ybcn and Y, resembling typical TTG. The amphibolite and tonalite gneiss show less fractionated REE patterns. SHRIMP zircon U-Pb dating on one gneissic trondhjemite, one amphibolite and one tonalite gave crystallization ages of 2667±21 Ma, 2701±10 Ma and 2697±9 Ma, respectively. They all recorded a metamorphic event at about 2.48 Ga. The SHRIMP zircon oxygen isotope analysis for a trondhjemite and an amphibolite gave δ18O values of 6.2±0.3‰ and 6.3±0.4‰, respectively. The oxygen isotope ratios higher than normal mantle values suggest a source experienced low temperature alteration. The laser fluoration analysis of bulk minerals gave δ18O values of 6.4-8.8‰ for zircon and 12.5-15.2‰ for quartz. The zircon Lu-Hf isotope analysis on the trondhjemite and amphibolite gave similar ɛHf(t) values of 0.08±0.48 and 0.07±0.63, respectively. Whole-rock ɛNd(t) values range from -1.5 to +1.0. These trondhjemite and tonalite can be interpreted as derivation from partial melting of subducted oceanic slab with a garnet-amphibolite residue.Considering the 2.67 Ga A-type granitic rocks at Huji in the interior of the craton, plate subduction took place in the northern edge of the Yangtze Craton. The Yudongzi trondhjemite and tonalite were formed in an active continental margin and the A-type granites formed as a response to the back-arc extension. The identification of 2.65-2.7 Ga subduction provides a possible link of the Yangtze Craton with the other cratons worldwide. This is also a new clue to reconstruct the Neoarchean supercontinent Kenorland.

Zircon geochronology and Hf-O isotope geochemistry from granites in the Iapetus Suture Zone in Ireland and the Isle of Man

NASA Astrophysics Data System (ADS)

Fritschle, Tobias; Daly, J. Stephen; Whitehouse, Martin J.; McConnell, Brian; Buhre, Stephan

2014-05-01

Late Caledonian syn- to post-orogenic granites located in the Iapetus Suture Zone (ISZ) in Ireland and Britain have been related to A-type subduction and possible slab breakoff [1] following the Laurentia-Avalonian collision. Lack of reliable age data (especially in Ireland) has inhibited petrogenetic investigations of these rocks. Hence, ion microprobe U-Pb and oxygen isotope analyses as well as LA-MC-ICPMS Lu-Hf isotopic measurements on zircons from Irish and Isle of Man granites have been undertaken to provide better constraints on this enigmatic episode of the Caledonian Orogeny. Four stages of Late Caledonian granitic magmatism (c. 435, 417, 410 and 394 Ma) are indicated by U-Pb dating of oscillatory-zoned magmatic zircons. The Crossdoney, Kentstown, Drogheda and Ballynamuddagh granites together with a rhyolite from Glenamaddy have yielded U-Pb concordia ages, interpreted as intrusion-ages, between 419.9 ± 4.3 Ma (Glenamaddy) and 415.8 ± 2.0 Ma (Crossdoney) with a weighted average of 417.5 ± 0.9 Ma (MSWD = 1.3). The Glenamaddy Granite - which intruded the Glenamaddy Rhyolite - yielded an age of 410 ± 2.1 Ma. In addition, the Rockabill Granite yielded a younger age of 393.9 ± 1.9 Ma, whereas the Carnsore Granite yielded an older age of 434.6 ± 1.9 Ma. Inherited zircons (487 to 453 Ma) occur in several of the granites, and are interpreted to have been derived from Ordovician arc magmatic rocks accreted within the ISZ. A younger group of c. 440 Ma inherited zircons occurs in the c. 417 Ma Crossdoney and Ballynamuddagh granites. These grains could be related to continued or renewed Silurian arc magmatism. Hf-O isotopic measurements on the dated zircon grains range between -2 and +7 ɛHfi units and 5.5 to 8.5 o δ18O. These are interpreted to indicate the contribution of juvenile mantle melts - possibly derived from the Ordovician arc - to some of the granites. Significant heterogeneities in zircon oxygen isotopes in at least four of the granites further suggest the involvement of isotopically distinct protoliths. The Dhoon and Foxdale granites in the Isle of Man were previously regarded to be of Late Caledonian age. Surprisingly, zircons from these granites yielded concordant U-Pb ages of 455.6 ± 2.1 Ma (Dhoon) and 455.9 ± 2.1 Ma (Foxdale), respectively. Inherited cores are of Meso- and Palaeoproterozoic age. Hafnium isotopic analyses of the dated magmatic zircon rims from both intrusions are slightly more radiogenic than those from the Late Caledonian granites. Their δ18O values range between 5.5 to 7.5 o. The Isle of Man granites are now suggested to be the plutonic equivalents of a Late Ordovician (Caradocian) volcanic arc generated in the Iapetus Ocean. [1] Atherton & Ghani (2002), Lithos 62, 65-85.

Exsolution lamellae as fast diffusion pathways in rutile: implications for U-Pb thermochronology and Zr thermometry

NASA Astrophysics Data System (ADS)

Smye, A.; Seman, S.; Roberts, N. M. W.; Condon, D. J.; Davis, B.

2017-12-01

Geophysical processes impart characteristic thermal signatures to the lithosphere. Near-continuous thermal histories can be obtained from inversion of intracrystalline U-Pb age profiles in rutile and apatite provided that it can be shown that profile formed in response to Fickian-type diffusion. Here, we present the results of a combined LA-ICPMS and ID-TIMS U-Pb study on rutile grains from two garnet-bearing granulite xenoliths from a kimberlite in the Archean Slave province. Interpreted using numerical models, we show that the rutile U-Pb isotope systematics are consistent with slow-cooling following crystallization at 1.2 Ga, contemporaneous with the Mackenzie dike swarm. However, inversion of rutile U-Pb age gradients is complicated by the ubiquitous presence of ilmenite exsolution lamellae. We show that these lamellae act as fast diffusion pathways for Pb and High Field Strength Elements, including Zr. This has important implications for the use of rutile as a U-Pb themochronometer and as a single-phase thermometer.

Reliance on (210)Pb chronology can compromise the inference of preindustrial Hg flux to lake sediments.

PubMed

Cooke, Colin A; Hobbs, William O; Michelutti, Neal; Wolfe, Alexander P

2010-03-15

Lake sediments are frequently used to reconstruct the rate and magnitude of human impacts on the biogeochemical cycle of mercury (Hg). The vast majority of these studies rely on excess (210)Pb inventories in short cores to temporally constrain recent trends in Hg deposition, revealing an approximately 3-fold increase in Hg deposition since preindustrial times. However, the exhaustion of unsupported (210)Pb and the onset of widespread global Hg pollution converge temporally in the late 19th century, raising the possibility that preindustrial Hg fluxes are poorly constrained. Here, we combine (210)Pb and accelerator mass spectrometry (AMS) (14)C dated lake sediment records from arctic and Andean lakes to assess the reliability of (210)Pb-derived chronologies in the estimation of preindustrial Hg fluxes. For all four studied lakes, relying on (210)Pb chronologies results in an overestimate of preindustrial Hg fluxes, because extrapolated basal (210)Pb sedimentation rates are systematically overestimated in comparison to accumulation models that include (14)C dates. In the Andes, the use of (14)C dates is critical toward assessing the full history of Hg pollution, which extends beyond the industrial era. In the Arctic, (14)C dating suggests that Hg deposition may have increased >10-fold since the Industrial Revolution, rather than the commonly quoted 3-fold increase. The incorporation of (14)C dates may therefore be necessary if accurate Hg flux histories are sought from oligotrophic lake sediments.

First 226Ra- 210Pb dating of a young speleothem

NASA Astrophysics Data System (ADS)

Condomines, M.; Rihs, S.

2006-10-01

Whereas the method based on the decrease of excess 210Pb has already been used to date young (< 120 yr) speleothems (e.g. [M. Baskaran, T. M. Iliffe, Age determination of recent cave deposits using excess 210Pb — A new technique, Geophys. Res. Lett. 20 (1993) 603-606.]), this paper presents the first dating of a speleothem through the 226Ra- 210Pb method. Dating of a young hydrothermal stalagmite from the Mt Cornadore cave (St Nectaire, French Massif Central) was made possible by the high 226Ra and negligible 210Pb contents of such carbonates, formed by precipitation from CO 2-rich thermal waters. ( 210Pb/ 226Ra) ratios regularly increase with depth along the axis of the 33 cm long stalagmite. The age-depth relationship can be interpreted by two main phases of growth, with high but variable axial growth rates of 5.3 mm/yr from 1909 to 1967, and 2.6 mm/yr from 1967 to 1989 (alternatively, the oldest phase can be subdivided in three episodes with growth rates varying from 2 to 7 mm/yr). Thin-section examination reveals the presence of numerous laminae, indicating infra-annual variations. We suggest that this fine layered structure might reflect short-term fluctuations in drip waters, possibly induced by near-surface mixing between thermal and ground waters, and ultimately linked to the pluviometry. A detailed examination of this laminated structure combined with 226Ra- 210Pb dating could thus provide a high-resolution record of local paleohydrological fluctuations.

Determinations of rare earth element abundance and U-Pb age of zircons using multispot laser ablation-inductively coupled plasma mass spectrometry.

PubMed

Yokoyama, Takaomi D; Suzuki, Toshihiro; Kon, Yoshiaki; Hirata, Takafumi

2011-12-01

We have developed a new calibration technique for multielement determination and U-Pb dating of zircon samples using laser ablation-inductively coupled plasma mass spectrometry (ICPMS) coupled with galvanometric optics. With the galvanometric optics, laser ablation of two or more sample materials could be achieved in very short time intervals (~10 ms). The resulting sample aerosols released from different ablation pits or different solid samples were mixed and homogenized within the sample cell and then transported into the ICP ion source. Multiple spot laser ablation enables spiking of analytes or internal standard elements directly into the solid samples, and therefore the standard addition calibration method can be applied for the determination of trace elements in solid samples. In this study, we have measured the rare earth element (REE) abundances of two zircon samples (Nancy 91500 and Prešovice) based on the standard addition technique, using a direct spiking of analytes through a multispot laser ablation of the glass standard material (NIST SRM612). The resulting REE abundance data show good agreement with previously reported values within analytical uncertainties achieved in this study (10% for most elements). Our experiments demonstrated that nonspectroscopic interferences on 14 REEs could be significantly reduced by the standard addition technique employed here. Another advantage of galvanometric devices is the accumulation of sample aerosol released from multiple spots. In this study we have measured the U-Pb age of a zircon sample (LMR) using an accumulation of sample aerosols released from 10 separate ablation pits of low diameters (~8 μm). The resulting (238)U-(206)Pb age data for the LMR zircons was 369 ± 64 Ma, which is in good agreement with previously reported age data (367.6 ± 1.5 Ma). (1) The data obtained here clearly demonstrate that the multiple spot laser ablation-ICPMS technique can become a powerful approach for elemental and isotopic ratio measurements in solid materials.

Constraining the 40K decay constant with 87Rb-87Sr - 40K-40Ca chronometer intercomparison

NASA Astrophysics Data System (ADS)

Naumenko-Dèzes, Maria O.; Nägler, Thomas F.; Mezger, Klaus; Villa, Igor M.

2018-01-01

A literature survey reveals that the K-Ar chronometer gives ages that are ca. 1% younger than U-Pb ages. This offset is generally attributed to an inaccurate 40K decay constant. Three geological samples selected from a shortlist of eight with known U-Pb ages were investigated using detailed petrological methods and subsequently the Rb-Sr and K-Ca chronometers in order (a) to evaluate if they meet the requirement of a geological history reflecting a ;point-like; event (i.e. isochronous formation and subsequent ideal closure of chronometers) and (b) to narrow down the systematic uncertainty on the 40K decay constant by investigating the metrologically traceable K-Ca decay branch. Lepidolite of the Rubikon pegmatite, Namibia, was dated with Rb-Sr at 504.7 ± 4.2 Ma and the phlogopite and apatite from the Phalaborwa carbonatite complex, South Africa, yielded a Rb-Sr age of 2058.9 ± 5.2 Ma. Both Rb-Sr ages agree with published U-Pb ages. The Rb-Sr age of the late Archean Siilinjärvi carbonatite, Finland, records a later regional metamorphic event at 1869 ± 10 Ma. Only the samples from the Phalaborwa complex represent a ;point-like; magmatic event and meet all the criteria to make them suitable for the 40K decay constant intercalibration. The Phalaborwa K-Ca isochron has a slope of 1.878 ± 0.012. Forcing the K-Ca isochron to coincide with the U-Pb and Rb-Sr ages gives one equation with two unknowns. Assuming that the branching ratio of the K-Ca branch, BCa, lies in the interval (k = 2) of all published references, 0.8925 < BCa < 0.8963, then the most reliable uncertainty interval (k = 2) for the total 40K decay constant, λtot, is calculated as 5.484 × 10-10 a-1 < λtot < 5.498 × 10-10 a-1. This confirms that the currently used IUGS recommendation is inaccurate.

Zircon U-Pb ages and Hf-O isotopic composition of migmatites from the Zanjan-Takab complex, NW Iran: Constraints on partial melting of metasediments

NASA Astrophysics Data System (ADS)

Moghadam, Hadi Shafaii; Li, Xian-Hua; Stern, Robert J.; Ghorbani, Ghasem; Bakhshizad, Farzaneh

2016-01-01

We study migmatites and other metamorphic rocks in the Zanjan-Takab region of NW Iran and use these results to report the first evidence of Oligocene core complex formation in Iran. Four samples of migmatites associated with paragneisses, including leucosomes and associated para-amphibolite melanosomes were selected for U-Pb dating and Hf-O isotopic analysis. Zircon cores - interpreted as originally detrital zircons - have variable ages that peak at ca. 100-110 Ma, but their sedimentation age - indicated by the youngest 206Pb/238U ages - is ca. 35-40 Ma. New zircons associated with incipient melting occur as overgrowths around zircon cores and/or as newly grown grains. Morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting. All four samples contain zircons with rims that yield 206Pb/238U ages of 28-25 Ma, indicating that partial melting occurred in Late Oligocene time. δ18O values for zircon rims vary between 8.2 and 12.3‰, significantly higher than expected for mantle inputs (δ18O 6‰) and consistent with equilibrium with surface materials. Zircon rims yield εHf(t) between 2.2 and 12.4 and two-stage Hf model ages of 448-562 Ma, indicating that the region is underlain by Cadomian-Caledonian crust. According to the Hf-O isotopic values, the main mechanism forming zircon rims was dissolution of pre-existing detrital zircons with reprecipitation of new zircon shortly thereafter. Oligocene ages indicate that partial melting accompanied core complex formation in the Zanjan-Takab region. Extension, melting, and core complex formation in south-central Iran are Eocene in age, but younger ages of Oligocene-Miocene in NW Iran and Turkey indicate that extension was distributed throughout the region during Cenozoic time.

Provenance analysis of the Late Paleozoic sedimentary rocks in the Xilinhot Terrane, NE China, and their tectonic implications

NASA Astrophysics Data System (ADS)

Han, Jie; Zhou, Jian-Bo; Wilde, Simon A.; Song, Min-Chun

2017-08-01

The Xilinhot Terrane is located in the eastern segment of the Central Asian Orogenic Belt in NE China, and is a key to a hotly debated issue on the Paleozoic tectonic evolution of this giant progenic belt. To constrain the tectonic evolution of the Xilinhot Terrane in the Late Paleozoic, we undertook zircon U-Pb dating and geochemical analyses of the Zhesi and Benbatu formations in the Suolun and Xi Ujimqin areas in the Xilinhot Terrane. Samples of the Benbatu Formation yield detrital zircon U-Pb ages ranging from 2659 Ma to 316 Ma, with four age populations at: 2659-1826 Ma, 1719-963 Ma, 590-402 Ma, and 396-316 Ma, whereas samples from the Zhesi Formation yield detrital zircon U-Pb ages ranging from 1967 Ma to 250 Ma, with four age populations at: 1967-1278 Ma, 971-693 Ma, 561-403 Ma, and 399-250 Ma. The age groups of both the Benbatu and Zhesi formations in the Xilinhot Terrane are similar to those in other parts of the Central Asian Orogenic Belt (CAOB). This evidence indicates that the Xilinhot Terrane is a microcontinent, and not an accretionary complex as previously thought. Furthermore, the youngest zircon grains in the Benbatu and Zhesi formations yield weighted mean 206Pb/238U ages of 322 ± 12 Ma (MSDW = 0.12, n = 4) and 257 ± 2.8 Ma (MSDW = 1.6, n = 8), respectively. Combined with fossil data, our new data suggest that the Benbatu and Zhesi formations in the Xilinhot Terrane were possibly deposited at ∼322 Ma and ∼257 Ma, respectively. Based on the provenance of the Carboniferous-Permian sandstones came from the blocks of NE China, we speculate that the Xilinhot Terrane is the western part of the Songliao block.

Age, temperature and pressure of metamorphism in the Tasriwine Ophiolite Complex, Sirwa, Morocco

NASA Astrophysics Data System (ADS)

Samson, S. D.; Inglis, J.; Hefferan, K. P.; Admou, H.; Saquaque, A.

2013-12-01

Sm-Nd garnet-whole rock geochronology and phase equilbria modeling have been used to determine the age and conditions of regional metamorphism within the Tasriwine ophiolite complex,Sirwa, Morocco. Pressure and temperature estimates obtained using a NaCaKFMASHT phase diagram (pseudosection) and garnet core and rim compositions predict that garnet growth began at ~0.72GPa and ~615°C and ended at ~0.8GPa and ~640°C. A bulk garnet Sm-Nd age of 645.6 × 1.6 Ma, calculated from a four point isochron that combines whole rock, garnet full dissolution and two successively more aggressive partial dissolutions, provides a precise date for garnet formation and regional metamorphism. The age is nearly 20 million years younger than a previous age estimate of regional metamorphism of 663 × 14 Ma based upon a SHRIMP U-Pb date from rims on zircon from the Irri migmatite. The new data provide further constraints on the age and nature of regional metamorphism in the Anti-Atlas mountains and emphasizes that garnet growth during regional metamorphism may not necessarily coincide with magmatism/anatexis which predominate the signature witnessed by previous U-Pb studies. The ability to couple PT estimates for garnet formation with high precision Sm- Nd geochronology highlights the utility of garnet studies for uncovering the detailed metamorphic history of the Anti-Atlas mountain belt.

Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite: Paleoproterozoic gold mineralization in northern Australia

NASA Astrophysics Data System (ADS)

Rasmussen, Birger; Sheppard, Stephen; Fletcher, Ian R.

2006-02-01

The inability to establish absolute ages for gold deposition in the Pine Creek orogen of northern Australia has led to conflicting ore deposit models, ranging from intrusion related, which predict that gold mineralization was synchronous with granite magmatism (ca. 1835 1820 Ma), to orogenic, which place ore deposition nearly 100 m.y. later. Here we present ion microprobe U-Pb geochronology for a mineralized quartz reef from Tom's Gully mine, Mount Bundey, Northern Territory, Australia, and nearby granitic rocks and associated contact aureoles. Isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca. 1825 Ma, whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207Pb/206Pb age of 1780 ± 10 Ma, interpreted as the time of gold mineralization. Mineralization therefore postdated intrusion by ˜45 m.y. and preceded a postulated ca. 1740 1730 Ma cratonwide orogenic gold event by ˜50 m.y. Hence, neither the intrusion-related model nor the recently proposed orogenic model is applicable. Combined with a reevaluation of age data from the nearby Goodall gold deposit, our data suggest that mineralization coincides with, and may be related to, an episode of regional low-grade metamorphism, deformation, and fluid circulation (Shoobridge event). Our results demonstrate the importance of high-precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits, and of testing the veracity of models for ore formation.

U, Th, and Pb isotopes in hot springs on the Juan de Fuca Ridge

NASA Technical Reports Server (NTRS)

Chen, J. H.

1987-01-01

Concentrations and isotopic compositions of U, Th, and Pb in three hydrothermal fluids from the Juan de Fuca Ridge were determined from samples obtained by the Alvin submersible. The samples were enriched in Pb and Th relative to deep-sea water, and were deficient in U. No clear relationship with Mg was found, suggesting nonideal mixing between the hot hydrothermal fluids and the cold ambient seawater. Values for U-234/U-238 have a seawater signature, and show a U-234 enrichment relative to the equilibrium value. The Pb isotopic composition has a uniform midocean ridge basalt signature, and it is suggested that Pb in these fluids may represent the best average value of the local oceanic crust.

Middle Eocene Climatic Optimum linked to continental arc flare-up in Iran?

NASA Astrophysics Data System (ADS)

van der Boon, A.; Kuiper, K.; van der Ploeg, R.; Cramwinckel, M.; Honarmand, M.; Sluijs, A.; Krijgsman, W.; Langereis, C. G.

2017-12-01

A 500 kyr episode of 3-5 °C gradual global climate warming, some 40 Myr ago, has been termed the Middle Eocene climatic optimum (MECO). It has been associated with a rise in atmospheric CO2 concentrations, but the source of this carbon remains enigmatic. We show, based on new Ar-Ar ages of volcanic rocks in Iran and Azerbaijan, that the time interval spanning the MECO was associated with a massive increase in continental arc volcanism. We also collected almost 300 Ar-Ar and U-Pb ages from literature. Typically, U-Pb ages from the Eocene are slightly younger, by 3 Myr, than Ar-Ar ages. We observed that U-Pb ages are obtained mostly from intrusive rocks and therefore must reflect an intrusive stage that post-dated extrusive volcanism. Combining all ages for extrusive rocks, we show that they cluster around 40.2 Ma, exactly within the time span of the MECO (40.5-40.0 Ma). We estimate volumes of volcanism based on a shapefile of outcrops and average thickness of the sequences. We calculate CO2 estimates using a relation volcanism-CO2 that was earlier used for the Deccan traps (Tobin et al., 2017). Our calculations indicate that the volume of the Iranian middle Eocene volcanic rocks (estimated at 37000 km3) is sufficient to explain the CO2 rise during the MECO. We conclude that continental arc flare-up in the Neotethys subduction zone is a plausible candidate for causing the MECO.

Changes in heavy metals in Antarctic snow from Coats Land since the mid-19th to the late-20th century

NASA Astrophysics Data System (ADS)

Planchon, Frédéric A. M.; Boutron, Claude F.; Barbante, Carlo; Cozzi, Giulio; Gaspari, Vania; Wolff, Eric W.; Ferrari, Christophe P.; Cescon, Paolo

2002-06-01

V, Cr, Mn, Cu, Zn, Co, Ag, Cd, Ba, Pb, Bi and U have been measured in a series of dated snow samples, covering the period from 1834 to 1990, collected at remote, low accumulation sites in Coats Land, Antarctica. They were determined by ultrasensitive inductively coupled sector field mass spectrometry in ultraclean conditions. Concentrations are found to be extremely low, down to 3×10 -15 g/g, for most metals, then confirming the high purity of Antarctic snow. The results show contrasting time trends for the different metals. For Mn, Co, Ba, and possibly V and Cd, no clear time trends are observed. For Cr, Cu, Zn, Ag, Pb, Bi and U, on the other hand, pronounced enhancements are observed during the recent decades. They are attributed to emissions of heavy metals to the atmosphere from human activities in Southern America, Southern Africa and Australia, especially non-ferrous metal mining and smelting in Chile, Peru, Zaire, Zambia and Australia. It shows that atmospheric pollution for heavy metals in the remote Antarctic continent is not limited to Pb and Cu, as previously thought, but also affects several other metals. It is a further indication that atmospheric pollution for heavy metals is really global.

Provenance of Modern Soils and Limestone and Chert Bedrock of Middle Tennessee Assessed Using Detrital Zircon U-Pb Geochronology

NASA Astrophysics Data System (ADS)

Ayers, J. C.; Katsiaficas, N. J.; Wang, X.

2014-12-01

Relatively thick soils mantle limestone bedrock throughout much of middle TN. Detrital zircon U-Pb geochronology was used to test two hypotheses: 1) That soil formed by accumulation of insoluble residue during chemical weathering of "dirty" limestone bedrock. 2) That an exotic component, perhaps wind-blown loess, was deposited and weathered to form soil. Samples of soil and underlying bedrock were collected from flat surfaces at the tops of cliffs. At Site 1 the Mississippian cherty limestone of the Fort Payne Formation was collected along with the B1 and B2 horizons of the overlying ultisol. At Site 2 a composite sample of A and B horizons of an alfisol and a sample of the underlying Ordovician limestone of the Hermitage Formation were collected. Zircon was recovered from soil and limestone samples, imaged using cathodoluminescence, and analyzed for trace elements and U-Pb isotopes using a 193 nm laser and quadrupole ICP-MS. Discordant analyses were discarded and 206Pb/238U ages are reported. Trace element concentrations and ratios in zircon seem to not be useful as provenance indicators. However, comparison of U-Pb age spectra showed that soils at both sites predominantly formed by weathering of limestone, with a small exotic component. The Hermitage has significant age peaks at ~1330, 1043, 955 and 439 Ma, and its overlying soil has age peaks at 1410, 1235, 1036 and 442 Ma. The age spectra are significantly different (Kolmogorov-Smirnov probability P = 0.01 < 0.05 significance). The Fort Payne has age peaks at ~1253, 967 and 417 Ma, while the B1 has age peaks at 1440, 1182, 1012 and 450 Ma (K-S P = 0.051) and the B2 at 1240, 941, 362, 81 and 33 Ma (K-S P = 0.073). The young ages in B2 require an exotic component that may account for ~25% of the measured ages. The source of the exotic material has not yet been identified, but its zircon age spectrum does not match previously published age spectra for the regional Pleistocene Peoria loess. Bedrock age peaks overlap with the Grenville, Taconic and Acadian orogenies of eastern North America. This study demonstrates that dating of detrital zircon is a powerful tool for determining the provenance of soil and limestone.

The 3.66 Ga Nuvvuagittuq Tonalite-Trondhjemite-Granodiorite Suite: A Case of Hadean Anatexis

NASA Astrophysics Data System (ADS)

Stevenson, R. K.; Bizzarro, M.

2011-12-01

The ca 3.8-4.3 Ga Nuvvuagittuq supracrustal sequence in northern Quebec, Canada, ranks as one of the most exciting early Earth terrain discoveries of the past fifteen years. The supracrustal sequence consists of mafic amphibolite rocks with rare felsic schists along with ultramafic sills and oxide-rich and quartz-rich iron formation. The supracrustal sequence is semi-oval in form and is folded around a core of foliated tonalite-trondhjemite-granodiorite (TTG) suite. A similar TTG suite forms a sheath around the exterior margin of the Nuvvuagittuq supracrustal sequence. Zircons from a felsic volcanic unit within the sequence yielded a minimum age of 3.8 Ga (U-Pb) for volcanic rocks of the sequence (David et al. 2009). A study of the Nd142 isotope systematics of the sequence found evidence for a Nd142 deficit in the sequence and a Nd142-Sm147 isochron yielded an age of 4.28 Ga (O'Neil et al. 2008). Dating of zircons from the tonalite sheath (U-Pb TIMS; David et al., 2009) and from the inner core (U-Pb SHRIMP; this study) yield similar ages (3.66 Ga). Zircons from this sequence undoubtedly contributed to the Hf isotopic composition of Archean sedimentary sequences of the Canadian Shield (Stevenson and Patchett 1990).The zircons from the TTG suite are very homogeneous and only have rare thin metamorphic rims that yield an age of 2.7 Ga (U-Pb Shrimp; this study). Radiogenic isotope studies (Sm-Nd and Lu-Hf) indicate that the TTG suite is an anatectic product of the melting of the Nuvvuagittuq supracrustal suite. The geochemical compositions of the Nuvvuagittuq TTG suite are compared with the compositions of ca 3.6 TTG suites from other localities around the world. David, J. Godin, L., Stevenson, R., O'Neil, J. and Francis, D. 2009. U-Pb ages (3.8-2.7 Ga) and Nd isotope data from the newly- identified Eoarchean Nuvvuagittuq supracrustal sequence, Superior Craton, Canada. GSA Bulletin 121; 150-163. O'Neil, J., Carlson, R.W., Francis, D., and Stevenson, R.K. 2008. Neodymium-142 Evidence Hadean Mafic Crust. Science 321 (Sept. 26), 1828-1831. Stevenson, R.K. and Patchett, P.J. (1990): Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons. Geochimica et Cosmochimica Acta 54, 1683-1697.

A tale of two cores: Evaluation of 210Pb dating methods of salt marsh sediments for two cores collected 30 years apart

NASA Astrophysics Data System (ADS)

Fuller, C.; Drexler, J. Z.

2016-12-01

210Pb dating of wetland sediments is commonly used to constrain recent C accumulation rates and contaminant input histories. However, uncertainties in 210Pb-derived rates and validation of accumulation and accretion rates using an independent tracer are often not reported. We describe here 210Pb and 137Cs profiles in two cores from a salt marsh in south San Francisco Bay, California, collected in 1981 and 2011 within 5 m of each other, to compare and evaluate 210Pb dating methods. In the 1981 core, unsupported 210Pb (210PbXS) was detected to 12 cm and yielded mass accumulation rates (MAR) of 0.043 and 0.036 g/cm2/y using the Constant Flux-Constant Sedimentation method (CF:CS) and Constant Rate of Supply (CRS) methods, respectively. Accretion rates (S) of 0.17 (CF:CS) and 0.12 cm/y (CRS) were calculated from these MARs. The distinct 137Cs peak at 4-6 cm in the 1981 core is in good agreement with the210Pb-based 1963 depth (3.4 and 4 cm, CF:CS and CRS, respectively). 210PbXS was detectable to 18 cm in the 2011 core, and yielded a CF:CS MAR (0.077 g/cm2/y; S = 0.35 cm/y) that is about two times greater than the mass-weighted average CRS MAR (0.044 g/cm2/y; S = 0.16 cm/y). Broad subsurface maxima in 137Cs and 239Pu were observed between 16 and 24 cm in the 2011 core, which are 5 to 11 cm deeper than the 1963 depth calculated by the 2011 and 1981 210Pb-derived MARs. The apparent migration and broadening of bomb-fallout radionuclide peaks over 30 years negates their use in validating 210Pb dating. Because of low 210PbXS activities in both cores, the base of the 210PbXS profile and integrated activity used in CRS are underestimated, resulting in the lower CRS MARs that decrease with increasing depth. The range of MARs determined for two cores within 5 m but separated by 30 years will be used as an example to evaluate the uncertainties that need to be reported with C accumulation rates and contaminant histories derived from 210Pb dating of sediment archives.

40Ar/39Ar geochronology of terrestrial pyroxene

NASA Astrophysics Data System (ADS)

Ware, Bryant; Jourdan, Fred

2018-06-01

Geochronological techniques such as U/Pb in zircon and baddeleyite and 40Ar/39Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40Ar/39Ar has been met with little success so far. In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40Ar/39Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2σ uncertainties of ±1.8-0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40Ar/39Ar plagioclase ages showing that 40Ar/39Ar dating of pyroxene is a suitable geochronological tool. Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40Ar/39Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40Ar/39Ar of pyroxene can be used as a powerful tool to date the formation age of mafic rocks affected by greenschist metamorphism and will likely play an important role in high temperature thermochronology.

Refined depositional history and dating of the Tongaporutuan reference section, north Taranaki, New Zealand: new volcanic ash U-Pb zircon ages, biostratigraphy and sedimentation rates

USGS Publications Warehouse

Maier, K.L.; Crundwell, Martin P.; Coble, Matthew A.; Kingsley-Smith, Peter R.; Graham, Stephan A.

2016-01-01

This study presents new radiometric ages from volcanic ash beds within a c. 1900 m thick, progradational, deep-water clastic slope succession of late Miocene age exposed along the north Taranaki coast of the North Island, New Zealand. The ash beds yield U–Pb zircon ages ranging from 10.63 ± 0.65 Ma to 8.97 ± 0.22 Ma. The new ages are compatible with and provide corroboration of New Zealand Tongaporutuan Stage planktic foraminiferal and bolboformid biostratigraphic events identified in the same section. The close accord between these two age datasets provides a stratigraphically consistent and coherent basis for examining margin evolution. The arrival of a prograding clastic wedge and ensuing upward shoaling is recorded by sedimentation rates c. 2000 m/Ma–1 that are an order of magnitude higher than sedimentation rates on the precursor deep basin floor. This outcrop study provides new constraints for interpreting analogous subsurface deposits in Taranaki Basin and complements the regional late Miocene biostratigraphic dating framework.

210Pb-dating of a lake sediment core from Lough Carra (Co. Mayo, western Ireland): use of paleolimnological data for chronology validation below the 210Pb dating horizon.

PubMed

O'Reilly, J; Vintró, L León; Mitchell, P I; Donohue, I; Leira, M; Hobbs, W; Irvine, K

2011-05-01

The chronologies and sediment accumulation rates for a lake sediment sequence from Lough Carra (Co. Mayo, western Ireland) were established by applying the constant initial concentration (CIC) and constant rate of supply (CRS) hypotheses to the measured (210)Pb(excess) profile. The resulting chronologies were validated using the artificial fallout radionuclides (137)Cs and (241)Am, which provide independent chronostratigraphic markers for the second half of the 20th century. The validity of extrapolating the derived CIC and CRS dates below the (210)Pb dating horizon using average sedimentation rates was investigated using supplementary paleolimnological information and historical data. Our data confirm that such an extrapolation is well justified at sites characterised by relatively stable sedimentation conditions. Copyright © 2010 Elsevier Ltd. All rights reserved.

U-Th-Pb isotopic systematics of lunar norite 78235

NASA Technical Reports Server (NTRS)

Premo, W. R.; Tatsumoto, M.

1991-01-01

A pristine high-Mg noritic cumulate thought to be relict deep-seated lunar crust is studied with an eye to obtaining evidence of initial Pb isotopic composition and U/Pb ratios of early lunar magma sources and possibly of a primary magma ocean. A leaching procedure was conducted on polymineralic separates to assure the removal of secondary Pb components. The Pb from leached separates do not form a linear trend on the Pb-Pb diagram, indicating open-system behavior either from mixtures of Pb or postcrystallization disturbances. Calculated initial Pb compositions and corresponding U-238/Pb-204 (mu) values are presented, with the assumption of reasonably precise radiometric ages from the literature for norite 78236. The results obtained support the contention that high-Mg suite rocks are coeval with the ferroan anorthosites, both being produced during the earliest stages of lunar evolution.

Tracking hydrothermal alteration and mineralization in rock-forming and accessory minerals from the Lyon Mountain Granite and related iron oxide apatite (IOA) ores from the Adirondack Mountains, New York State

NASA Astrophysics Data System (ADS)

Buchanan, A.; Hanchar, J. M.; Steele-MacInnis, M. J.; Crowley, J. L.; Valley, P. M.; Fisher, C. M.; Fedo, C.; Piccoli, P. M.; Fournelle, J.

2012-12-01

The Lyon Mountain granite (LMG) is located in the northeastern Adirondack Mountains in New York State and hosts several low-titanium iron oxide apatite (IOA) ore deposits. The ores are predominately hosted by perthite bearing granite, which has been extensively altered to albite and microcline granite by Na and K metasomatism. This alteration results in several distinct groups of rocks that are dominated by either K or Na addition and a group composed of mixed Na and K addition. The different groups of altered perthite also lie on a trend suggestive of addition of Fe to each, consistent with a secondary mineralization origin. Previous work showed that the host rocks of the IOA ores have zircon with ~1150 Ma cores and 1060-1050 Ma rims and whole grains. This study aims to further constrain the timing of LMG emplacement, subsequent hydrothermal alteration, and Fe mineralization through geochemical analysis of the major, minor, and accessory phases and geochronology of accessory phases. SIMS analyses of zircon from several of the IOA ores reveal at least two periods of growth after LMG magmatism, at 1039 +/- 4.4 Ma and 1016 +/- 7 Ma to 1000 +/- 9 Ma. In situ EMPA and LA-ICPMS trace element analyses of the zircon rims and cores reveal that in two samples the zircon rims are enriched in rare earth elements (REE) compared to their cores, potentially pointing to a hydrothermal origin. Apatite has unusually high REE and Y concentrations (some total REE2O3 > 20 wt. % oxide and up to 8 wt. % oxide Y2O3), as does titanite, which allowed for the in situ analysis of Sm-Nd in apatite and titanite by LA-MC-ICP-MS. Initial Nd isotopic composition of both ore and host rock apatite and host rock titanite are consistent with published Adirondack initial Nd whole rock data, suggesting a local source for REE in these ores. EMPA and LA-ICPMS trace-element analyses of the major rock-forming minerals indicate that the feldspar have undergone Na-metasomatism and are depleted in REEs, perhaps signifying the "local source" and the mechanism of the REE enrichment in the LMG apatite in the IOA ores and host rocks. In contrast, the minor- and trace-element compositions of the other major rock-forming minerals (e.g., clinopyroxene and fayalite) as well as the zircon, and fluorite in the LMG have average igneous granitic trace- and minor-element compositions. To better understand the timing and origin of these post ~1050 Ma events, U-Pb ID-TIMS dating of apatite and titanite, and in situ LA-MC-ICPMS Sm-Nd analysis were done on the ore and host rock samples. Apatite dates range from 1050 to 850 Ma and titanite dates range from ~1015 to 970 Ma. There is significant age variation within samples and within grains. Titanite does not have sufficient spread for accurate Sm-Nd isochron dating and two ore-apatite samples have homogenous initial Nd isotopic and Sm-Nd elemental ratios, precluding calculation of Sm-Nd dates. A third ore sample shows a large spread in Sm-Nd and yields a Sm-Nd isochron date of ~850 Ma, in close agreement with U-Pb apatite dates. The Sm-Nd isochron and U-Pb apatite dates may reflect cooling recorded in these minerals or a younger hydrothermal mineralization event.

The timing and mechanism of depletion in Lewisian granulites

NASA Technical Reports Server (NTRS)

Cohen, A. S.; Onions, R. K.; Ohara, M. J.

1988-01-01

Large Ion Lithophile (LIL) depletion in Lewisian granulites is discussed. Severe depletions in U, Th, and other LIL have been well documented in Lewisan mafic and felsic gneisses, but new Pb isotopic analyses show little or no depletion in lithologies with high solidus temperatures, such as peridotite. This suggests that LIL transport in this terrane took place by removal of partial melts rather than by pervasive flooding with externally derived CO2. The Pb and Nd isotopic data gathered on these rocks show that the depletion and granulite metamorphism are distinct events about 250 Ma apart. Both fluid inclusions and cation exchange geothermometers date from the later metamorphic event and therefore have little bearing on the depletion event, suggesting a note of caution for interpretations of other granulite terranes.

Assessment of Alternative [U] and [Th] Zircon Standards for SIMS

NASA Astrophysics Data System (ADS)

Monteleone, B. D.; van Soest, M. C.; Hodges, K.; Moore, G. M.; Boyce, J. W.; Hervig, R. L.

2009-12-01

The quality of in situ (U-Th)/He zircon dates is dependent upon the accuracy and precision of spatially distributed [U] and [Th] measurements on often complexly zoned zircon crystals. Natural zircon standards for SIMS traditionally have been used to obtain precise U-Pb ages rather than precise U and Th concentration. [U] and [Th] distributions within even the most homogeneous U-Pb age standards are not sufficient to make good microbeam standards (i.e., yield good precision: 2σ < 5%) for (U-Th)/He dates. In the absence of sufficiently homogeneous natural zircon crystals, we evaluate the use of the NIST 610 glass standard and a synthetic polycrystalline solid “zircon synrock” made by powdering and pressing natural zircon crystals at 2 GPa and 1100°C within a 13 mm piston cylinder for 24 hours. SIMS energy spectra and multiple spot analyses help assess the matrix-dependence of secondary ion emission and [U] and [Th] homogeneity of these materials. Although spot analyses on NIST 610 glass yielded spatially consistent ratios of 238U/30Si and 232Th/30Si (2σ = 2%, n = 14), comparison of energy spectra collected on glass and zircon reveal significant differences in U, UO, Th, and ThO ion intensities over the range of initial kinetic energies commonly used for trace element analyses. Computing [U] and [Th] in zircon using NIST glass yields concentrations that vary by more than 10% for [U] and [Th], depending on the initial kinetic energy and ion mass (elemental, oxide, or sum of elemental and oxide) used for the analysis. The observed effect of chemistry on secondary ion energy spectra suggests that NIST glass cannot be used as a standard for trace [U] and [Th] in zircon without a correction factor (presently unknown). Energy spectra of the zircon synrock are similar to those of natural zircon, suggesting matrix compatibility and therefore potential for accurate standardization. Spot analyses on the zircon powder pellets, however, show that adequate homogeneity of [U] and [Th] (2σ = 37% and 33% for 238U/30Si and 232Th/30Si, respectively, n = 8) has yet to be achieved. Modeling shows that homogenization of [U] and [Th] within these pellets requires preparation of powders with <2 micron sized particles, which has yet to be achieved in sample preparation. Thus, the zircon synrock pellet remains a viable potential [U], [Th] standard, although the preparation of a sufficiently fine grained, homogeneous pellet is a work in progress.

The geological processes time scale of the Ingozersky block TTG complex (Kola Peninsula)

NASA Astrophysics Data System (ADS)

Nitkina, Elena

2013-04-01

Ingozersky block located in the Tersky Terrane of the Kola Peninsula is composed of Archean gneisses and granitoids [1; 5; 8]. The Archaean basement complexes on the regional geological maps have called tonalite-trondemit-gneisses (TTG) complexes [6]. In the previous studies [1; 3; 4; 5; 7] within Ingozersky block the following types of rocks were established: biotite, biotite-amphibole, amphibole-biotite gneisses, granites, granodiorites and pegmatites [2]. In the rocks of the complex following corresponding sequence of endogenous processes observed (based on [5]): stage 1 - the biotitic gneisses formation; 2 - the introduction of dikes of basic rocks; 3 phase - deformation and foliation; 4 stage - implementation bodies of granite and migmatization; 5 stage - implementation of large pegmatite bodies; stage 6 - the formation of differently pegmatite and granite veins of low power, with and without garnet; stage 7 - quartz veins. Previous U-Pb isotopic dating of the samples was done for biotite gneisses, amphibole-biotite gneisses and biotite-amphibole gneisses. Thus, some Sm-Nd TDM ages are 3613 Ma - biotite gnesses, 2596 Ma - amphibole-biotite gnesses and 3493 Ma biotite-amphibole gneisses.. U-Pb ages of the metamorphism processes in the TTG complex are obtained: 2697±9 Ma - for the biotite gneiss, 2725±2 and 2667±7 Ma - for the amphibole-biotite gneisses, and 2727±5 Ma for the biotite-amphibole gneisses. The age defined for the biotite gneisses by using single zircon dating to be about 3149±46 Ma corresponds to the time of the gneisses protolith formation. The purpose of these studies is the age establishing of granite and pegmatite bodies emplacement and finding a geological processes time scale of the Ingozerskom block. Preliminary U-Pb isotopic dating of zircon and other accessory minerals were held for granites - 2615±8 Ma, migmatites - 2549±30 Ma and veined granites - 1644±7 Ma. As a result of the isotope U-Pb dating of the different Ingozerskogo TTG complex rocks, the following age-formation stages are determined: protolith of the biotite gneisses - 3149±46 Ma; metamorphism, deformation of rocks, foliation - 2727±5 - 2725±2 - 2697±9 - 2667±7 Ma, granite bodies formation - 2615±8 Ma and biotite gneisses migmatization - 2549±30 Ma, formation of different pegmatite and granite veins -1644±7 Ma. Author are grateful to Akad. Mitrofanov F.P. and Bayanova T.B. for the consultations. The work is supported by RFBR 12-05-31063, 11-05-00570. 1.Batieva I.D., Belkov I.V. Granitoidnie formacii Kolskogo poluostrova. // Ocgerki po petrologiy, mineralogiy i metallogeniy Kolskogo poluostrova. L.: Nauka. 1968. p. 5-143. (in russian) 2. Belkov I.V., Zagorodny V.G., Predovsky A.A. et al. Stratigraficheskoe raschlenenie i korrelyacia dokembria severo-vostochoi chasty Baltiyskogo shita. L.: Nauka. 1971. p. 141-150. (in russian) 3. Docembriskaya tektonica severo-vostochoi chasty Baltiyskogo shita (Ob'asnitelnaya zapiska k tektonicheskoi karte severo-vostochoi chasty Baltiyskogo shita 1:500000) / ed.: F.P.Mitrofanov. Apatity: KFAN SSSR. 1992. 112 P. (in russian) 4. Zagorodny V.G., Radchenko A.T. Tectonika i glubinnoe stroenie severo-vostochoi chasty Baltiyskogo shita. Apatity: KFA SSSR. 1978. p. 3-12. (in russian) 5. Kozlov N.E., Sorohtin N.O., Glaznev V.N. et al. Geologia Arhea Baltiskogo shita. S.Pb.: Nauka. 2006. 329 p. (in russian) 6. Mitrofanov F.P. Sovremennie problemy i nekotorie resheniya dokembriskoy geologii kratonov. (2001) Litosphera.2001. V 1. P. 5-14. (in russian) 7. Ob'asnitelnaya zapiska k geologicheskoy karte severo-vostochoi chasty Baltiyskogo shita 1:500000 / ed.: F.P.Mitrofanov. Apatity: KFAN SSSR. 1994. 95 P. (in russian) 8. Haritonov L.Y. Structura i stratigraphia karelid vostoka Baltiskogo shita. M.: Nedra. 1966. 354 P. (in russian)

Ion microprobe mass analysis of lunar samples. Lunar sample program

NASA Technical Reports Server (NTRS)

Anderson, C. A.; Hinthorne, J. R.

1971-01-01

Mass analyses of selected minerals, glasses and soil particles of lunar, meteoritic and terrestrial rocks have been made with the ion microprobe mass analyzer. Major, minor and trace element concentrations have been determined in situ in major and accessory mineral phases in polished rock thin sections. The Pb isotope ratios have been measured in U and Th bearing accessory minerals to yield radiometric age dates and heavy volatile elements have been sought on the surfaces of free particles from Apollo soil samples.

Uranium-lead isotopic ages from the Sierra Nevada Batholith, California

NASA Astrophysics Data System (ADS)

Chen, James H.; Moore, James G.

1982-06-01

This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U-Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc-alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U-Pb ages of the Giant Forest-alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U-Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U-Pb, sphene U-Pb, hornblende K-Ar, and biotite K-Ar suggests that zircon U-Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U-Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K-Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.

Uranium-lead isotopic ages from the Sierra Nevada Batholith, California

USGS Publications Warehouse

Chen, J.

1982-01-01

This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U-Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc-alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U-Pb ages of the Giant Forest-alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U-Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U-Pb, sphene U-Pb, hornblende K-Ar, and biotite K-Ar suggests that zircon U-Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U-Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K-Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.

High-precision U-Pb geochronology of the Jurassic Yanliao Biota from Jianchang (western Liaoning Province, China): Age constraints on the rise of feathered dinosaurs and eutherian mammals

NASA Astrophysics Data System (ADS)

Chu, Zhuyin; He, Huaiyu; Ramezani, Jahandar; Bowring, Samuel A.; Hu, Dongyu; Zhang, Lijun; Zheng, Shaolin; Wang, Xiaolin; Zhou, Zhonghe; Deng, Chenglong; Guo, Jinghui

2016-10-01

The Yanliao Biota of northeastern China comprises the oldest feathered dinosaurs, transitional pterosaurs, as well as the earliest eutherian mammals, multituberculate mammals, and new euharamiyidan species that are key elements of the Mesozoic biotic record. Recent discovery of the Yanliao Biota in the Daxishan section near the town of Linglongta, Jianchang County in western Liaoning Province have greatly enhanced our knowledge of the transition from dinosaurs to birds, primitive to derived pterosaurs, and the early evolution of mammals. Nevertheless, fundamental questions regarding the correlation of fossil-bearing strata, rates of dinosaur and mammalian evolution, and their relationship to environmental change in deep time remain unresolved due to the paucity of precise and accurate temporal constraints. These limitations underscore the importance of placing the rich fossil record of Jianchang within a high-resolution chronostratigraphic framework that has thus far been hampered by the relatively low precision of in situ radioisotopic dating techniques. Here we present high-precision U-Pb zircon geochronology by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) from three interstratified ash beds previously dated by secondary-ion mass spectrometry (SIMS) technique. The results constrain the key fossil horizons of the Daxishan section to an interval spanning 160.89 to 160.25 Ma with 2σ analytical uncertainties that range from ±46 to ±69 kyr. These data place the Yanliao Biota from Jianchang in the Oxfordian Stage of the Late Jurassic, and mark the Daxishan section as the site of Earth's oldest precisely dated feathered dinosaurs and eutherian mammals.

Age of the granitic magmatism and the W-Mo mineralization in skarns of the Seridó belt (NE Brazil) based on zircon U-Pb (SHRIMP) and molybdenite Re-Os dating

NASA Astrophysics Data System (ADS)

Hollanda, Maria Helena B. M. de; Souza Neto, João A.; Archanjo, Carlos J.; Stein, Holly; Maia, Ana C. S.

2017-11-01

Over five hundred W-Mo skarns have been reported in the Neoproterozoic Seridó belt in the northeastern Brazil. The origin of these mineralizations has been attributed to metasomatic reactions occuring after the infiltration of hydrothermal fluids that are mostly derived from the plutonic magmatic activity that ranged between approximately 600 and 525 Ma. Here we date molybdenite using N-TIMS on Re-Os analysis of three major scheelite deposits (Brejuí, Bonfim and Bodó) hosted in the skarn horizons of the metasedimentary sequence. Molybdenite is an integral part of the mineralizations that include scheelite in skarns and, in the Bonfim deposit, gold concentrate in late brittle faults. The Re-Os ages are 554 ± 2 Ma (Brejuí), 524 ± 2 Ma (Bonfim) and 510 ± 2 Ma (Bodó). The age of the Brejuí molybdenite, however, appears to be anomalous based on the local geology of the deposit, which is located next to the contact of a batholith dated ca. 575 Ma. In turn, the Bonfim molybdenite yields similar ages in replicated samples with variable high Re contents. New U-Pb SHRIMP ages of four biotite (leuco)granite plutons vary from 577 ± 5 Ma to 526 ± 8 Ma, which overlap with molybdenite crystallization. These results indicate a close connection between the W-Mo mineralizations and the plutonic activity that intruded the belt after the peak HT/LP metamorphism. The latest pulses of felsic magmatism, which were contemporaneous with the emplacement of Be-Ta-Nb-Li pegmatites, therefore constitute a potential guide in the Seridó belt for prospective W-Mo deposits.

2.9, 2.36, and 1.96 Ga zircons in orthogneiss south of the Red River shear zone in Viet Nam: evidence from SHRIMP U-Pb dating and tectonothermal implications

NASA Astrophysics Data System (ADS)

Nam, Tran Ngoc; Toriumi, Mitsuhiro; Sano, Yuji; Terada, Kentaro; Thang, Ta Trong

2003-05-01

Orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the Day Nui Con Voi-Red River shear zone in northern Viet Nam. The orthogneiss complex has given some radiogenic dates of Early Proterozoic and Late Archean, which are the oldest ages ever registered for the Southeast Asian continent. Zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high-grade tectonothermal events in the complex. Sixty-five SHRIMP U-Th-Pb analyses of these zircons define three age groups of 2.84-2.91, 2.36, and 1.96 Ga. The age groups correspond to three periods of zircon generation. The oldest ˜2.9 Ga cores indicate a minimum age for the protolith of the orthogneiss complex. Two younger generations (including ˜2.36 Ga outer-cores and ˜1.96 Ga rims) probably grew during later high-grade tectono-metamorphic events, which were previously suggested by K-Ar and 40Ar/ 39Ar cooling ages of ˜2.0 Ga for synkinematic hornblendes. An early thermal history of the orthogneiss complex has been constrained, including a primary magma-crystallization stage starting at ˜2.9 Ga, followed by two Early Proterozoic (˜2.36 and ˜1.96 Ga) high-grade tectonothermal events. The ca. 2.9 Ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of Archean rocks in Indochina, and clearly indicates that the crustal evolution of northern Viet Nam started as early as Late Archean time.

What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

NASA Astrophysics Data System (ADS)

Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; van Hinsbergen, Douwe J. J.; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul

2015-03-01

The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-Pb zircon dates of 52 Ma for the upper Linzizong, and 40Ar/39Ar dates ( 51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of 5°N, compared to the upper part ( 20°N) and to underlying Cretaceous strata ( 20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by 52 Ma at 20°N.

When Did Midcontinent Rift Volcanism End and Where Was Laurentia at that Time?

NASA Astrophysics Data System (ADS)

Fairchild, L. M.; Swanson-Hysell, N.; Ramezani, J.; Sprain, C. J.; Gaastra, K. M.; Bowring, S. A.

2015-12-01

Data from the North American Midcontinent Rift provide a robust record of Laurentia's (cratonic North America's) paleogeographic position from ca. 1110 to 1080 Ma. The resulting apparent polar wander path (APWP) reveals rapid motion of the continent towards the equator throughout the rift's lifetime. Constraints on the age of the youngest volcanics within the rift and on the paleolatitude of Laurentia at that time are important for quantifying the rate of this motion and its apparent deceleration in the late stage of rift development. Furthermore, precise calibration of the APWP enhances the robustness of paleogeographic reconstructions. The three rift successions with ca. 1090 to 1085 Ma late stage volcanics are the Lake Shore Traps of Michigan, the Michipicoten Island Formation of Ontario and the Schroeder-Lutsen basalts of Minnesota. In past studies, paleomagnetic data from the Schroeder-Lutsen basalts have been grouped with results from the North Shore Volcanic Group, which it unconformably overlies. In this study, we separate these data and add newly developed results from 40 additional flows. New data from the Michipicoten Island Formation allow for a well constrained pole that now includes data from more than 25 flows. High quality paleomagnetic data are published for the Lake Shore Traps, and we complement these with a newly developed high precision U-Pb zircon date as an update to current constraints. Taken altogether, these data strengthen our understanding of the rift's demise and the rate of Laurentia's motion as rift volcanism gave way to post-rift sedimentation.

210Pb-226Ra and Other U-Series Disequilibria in Very Young MORB and Loihi Tholeiites

NASA Astrophysics Data System (ADS)

van der Zander, I.; Rubin, K. H.; Smith, M.; Perfit, M.; Bergmanis, E. C.

2003-12-01

Direct observations of submarine volcanic eruptions are very sparse. Radiometric age constraints on submarine lava flows are thus an essential component for understanding even the most recent histories of oceanic crust formation. Chronometers in the decadal to century time frame have heretofore been lacking. This study focuses on the development and application of 210Pb-226Ra disequilibria as a geochronometer to provide quantitative eruption age constraints over the past 100 years, using submersible-collected samples from the North Cleft segment of the Juan de Fuca Ridge (JDFR) and adjacent Axial seamount, 9° 50'N East Pacific Rise (EPR) and Loihi (Hawaii), areas with known stratigraphic field relations between mapped lava flows. The data set provides a unique opportunity to calibrate the 210Pb-226Ra geochronometer because it represents a broad selection of "zero age" (210Po-210Pb dated) and near-zero-age submarine lavas (glasses). 238U-230Th-226Ra-210Pb radioactive disequilibria in these samples will be discussed to rationalize the range of conditions responsible for producing 210Pb-226Ra disequilibria (effective half life = 22 yrs) as an initial step towards using this signature to constrain eruption ages and petrogenetic time scales. We will also investigate the temporal aspects of petrogenetic conditions responsible for producing the other disequilibria in these rocks, without the uncertainty imposed by decay corrections for rocks of unknown age. These data augment those we previously reported from the Aldo-Kihi and neighboring lava flows at 17° 26'S EPR (Rubin et al., EOS, 82, F1279, 2001). Systematic differences between and within study areas exist: most normal zero age MORB display modest 210Pb deficits (3-10%); older MORB (the N-cleft sheet flow) have smaller deficits (0-3%); rocks erupted in 1998 from Axial seamount have 8-15% 210Pb excesses; and, rocks erupted in 1996 at Loihi are in equilibrium within error. Disequilibria amongst other nuclides are all within the range of previously published values. Data broadly fall within the negatively correlated array of (226Ra/230Th) and (230Th/238U) (Sims et al., GCA, 66, 2002), yet within each geographic locale data are positively correlated. Although (210Pb/226Ra) covaries with some major/trace element signatures of low pressure crystal fractionation, it also covaries negatively with (226Ra/230Th) in all but the Loihi samples, raising the intriguing possibility that some or all of the 210Pb-226Ra disequilibria arises deeper within the mantle (i.e., during melting or transport. This would shorten dramatically the time scales over which those processes occur). The suite of petrogenetic variations will be examined in the context of existing models for basaltic melt genesis.

Isotope Sm-Nd age of the paleoproterozoic PGE-bearing Monchetundra massif trachytoid gabbronorites (Fennoscandian shield)

NASA Astrophysics Data System (ADS)

Kunakkuzin, Evgeniy; Bayanova, Tamara; Serov, Pavel; Borisenko, Elena

2015-04-01

Monchetundra massif is located in the central part of the Kola Peninsula (Russia) and it is the south-eastern part of the Main Ridge Intrusion. Monchetundra massif together with well-known layered mafic-ultramafic PGE-bearing intrusions in the Fennoscandian shield such as the Fedorovo-Pansky complex, the mt. Generalskaya, the Monchepluton is of interest as a target for the PGE prospecting (Mitrofanov et al. 2006; Nerovich et al., 2009; Grokhovskaya et al., 2003). According to some previously researchers (Nazimova, Rayan, 2008, Nerovich et al., 2009, Layered intrusions…p.1, 2004) rocks of the Monchetundra massif is subdivided into two to five syngenetic zones. Hence the last isotope-geochronological and isotope-geochemical data revealed that the massif includes at least four groups of mafic rocks distinguished by formation ages (Bayanova et al., 2010). The aim of this work is to present Sm-Nd dating results of trachytoid gabbronorites, which are the second mafic rocks group in the Monchetundra massif. The Sm-Nd investigations for these rocks were carried out for the first time. The second group of mafic rocks comprises of medium-grained and coarse-grained mesocratic gabbronorites of trachytoid texture, with they characterized by well-preserved primary magmatic minerals and gabbro-ophitic texture. The U-Pb ages on single zircon-baddeleyite for these rocks recently obtained (2505 ± 6 Ma, 2501 ± 8 Ma, 2504.4±2.7 Ma and 2507.5±7.7 Ma (Layered intrusions…p.1., 2004, Borisenko et al., 2013)). Two samples of trachytoid gabbronorites were selected to study these rocks by Sm-Nd isotopic method. Mineral isochrons plotted from plagioclase, ortho- and clinopyroxene and whole-rock minerals gave ages of 2496±27 (MSWD = 0.9; ɛNd = -1.6±0.5) and 2492±55 Ma (MSWD = 0.5; ɛNd = -1.7±0.5). The new Sm-Nd ages obtained are close to the U-Pb data on zircons and baddeleyites for this rocks group and consider as oridin of second mafic rocks group. All investigations are devoted to memory of academician RAS, professor F. Mitrofanov (Russia), he was a leader of scientific school for geology, geochemistry and metallogeny of ore deposits. The researches are conducted with the financial support of RFBR 13-05-00493, OFI-M 13-05-12055, IGCP-SIDA 599.

Mesozoic strike-slip movement of the Dunhua-Mishan Fault Zone in NE China: A response to oceanic plate subduction

NASA Astrophysics Data System (ADS)

Liu, Cheng; Zhu, Guang; Zhang, Shuai; Gu, Chengchuan; Li, Yunjian; Su, Nan; Xiao, Shiye

2018-01-01

The NE-striking Dunhua-Mishan Fault Zone (DMFZ) is one of two branches of the continental-scale sinistral Tan-Lu Fault Zone in NE China. The field data presented here indicate that the ca. 1000 km long DMFZ records two phases of sinistral faulting. The structures produced by these two phases of faulting include NE-SW-striking ductile shear belts and brittle faults, respectively. Mylonite-hosted microstructures and quartz c-axis fabrics suggest deformation temperatures of 450 °C-500 °C for the ductile shear belts. Combining new zircon U-Pb dates for 14 igneous rock samples analyzed during this study with the geology of this region indicates these shear belts formed during the earliest Early Cretaceous. This phase of sinistral displacement represents the initial formation of the DMFZ in response to the northward propagation of the Tan-Lu Fault Zone into NE China. A phase of Early Cretaceous rifting was followed by a second phase of sinistral faulting at 102-96 Ma, as evidenced by our new U-Pb ages for associated igneous rocks. Combining our new data with the results of previous research indicates that the DFMZ records a four-stage Cretaceous evolutionary history, where initial sinistral faulting at the beginning of the Early Cretaceous gave way to rifting during the rest of the Early Cretaceous. This was followed by a second phase of sinistral faulting at the beginning of the Late Cretaceous and a second phase of local rifting during the rest of the Late Cretaceous. The Cretaceous evolution of the DMFZ records the synchronous tectonic evolution of the NE China continent bordering the Pacific Ocean. Two phases of regional N-S compression generated the two phases of sinistral faulting within the DMFZ, whereas two-stage regional extension generated the two phases of rifting. The two compressive events were the result of the rapid low-angle subduction of the Izanagi and Pacific plates, whereas the two-stage extension was caused by the roll-back of these respective plates. The final closure of the Mongol-Okhotsk Ocean at the beginning of the Early Cretaceous intensified the synchronous compression in NE China, causing the northward propagation of the Tan-Lu Fault Zone.

Two mineralization events in the Baiyinnuoer Zn-Pb deposit in Inner Mongolia, China: Evidence from field observations, S-Pb isotopic compositions and U-Pb zircon ages

NASA Astrophysics Data System (ADS)

Jiang, Si-Hong; Chen, Chun-Liang; Bagas, Leon; Liu, Yuan; Han, Ning; Kang, Huan; Wang, Ze-Hai

2017-08-01

The Xing-Mong Orogenic Belt (XMOB) is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and has experienced multiple tectonic events. The Baiyinnuoer Pb-Zn deposit may be a rare case that documents two periods of mineralization in the tectonically complex XMOB. There are two types of Pb-Zn mineralization in the deposit: (1) skarn-type ore, hosted by the skarn in the contact zone between marble and granodiorite and within the marble and (2) vein-type ore, hosted by crystal tuff and feldspar porphyry. This study revealed that the host rocks, mineral assemblages, mineralization occurrences, S-Pb isotopes, and ages between the two types of ore are notably different. Zircon U-Pb dating indicates that the granodiorite was emplaced in the Early Triassic (244 ± 1 to 242 ± 1 Ma), the crystal tuff was deposited in the Early Cretaceous (140 ± 1 to 136 ± 1 Ma), and the feldspar porphyry was intruded in the Early Cretaceous (138 ± 2 to 136 ± 2 Ma). The first skarn mineralization occurred at ∼240 Ma and the second vein-type Pb-Zn mineralization took place between 136 and 129 Ma. Thus the Triassic orebodies were overprinted by Early Cretaceous mineralization. The sphalerite and galena from the skarn mineralization have higher δ34S values (-4.7 to +0.3‰) than the sphalerite, galena and aresenopyrite from the vein-type mineralization (-7.5 to -4.2‰), indicating different sulfur sources or ore-forming processes for the two types of mineralization. The Pb isotopic compositions of the two types of ore are very similar, suggesting similar lead sources. Geochemistry and Nd-Pb-Hf isotopic systematics of the igneous rocks in the region show that the Triassic granodiorite was generated from hybridization of mafic and felsic magmas due to strong crust-mantle interaction under the collisional setting that resulted following the closure of the Paleo-Asian Ocean and the collision of North China and Siberian cratons at the end of the Permian; while the Cretaceous igneous rocks at Baiyinnuoer originated from the partial melting of a juvenile lower crust with minor input from the crust caused by the underplating of mafic magma in an extensional setting.

U-Th-Pb systematics of the Estherville mesosiderite

NASA Technical Reports Server (NTRS)

Brouxel, M.; Tatsumoto, M.

1990-01-01

Results are presented on a detailed U-Th-Pb systematics of the Estherville mososiderite, which was performed in a study involving stepwise leaching experiments. The Pb-Pb internal 'isochrons' for Estherville yielded ages of 4556 + or - 35 Ma, 4506 + or - 75 Ma, and 4422 + or - 50 Ma, indicating that the silicate fraction of the Estherville mesosiderite is very heterogeneous and was formed early in the solar system history. Results clearly identifies the Pb-Pb isochron as a mixing line. The U-Pb lower-intercept ages could be divided into two groups: (1) around 3 Ga, and likely related to the 3.6 Ga heating event, and (2) close to 0 Ma and to 62 Ma.

Seawater 234U/238U recorded by modern and fossil corals

NASA Astrophysics Data System (ADS)

Chutcharavan, Peter M.; Dutton, Andrea; Ellwood, Michael J.

2018-03-01

U-series dating of corals is a crucial tool for generating absolute chronologies of Late Quaternary sea-level change and calibrating the radiocarbon timescale. Unfortunately, coralline aragonite is susceptible to post-depositional alteration of its primary geochemistry. One screening technique used to identify unaltered corals relies on the back-calculation of initial 234U/238U activity (δ234Ui) at the time of coral growth and implicitly assumes that seawater δ234U has remained constant during the Late Quaternary. Here, we test this assumption using the most comprehensive compilation to date of coral U-series measurements. Unlike previous compilations, this study normalizes U-series measurements to the same decay constants and corrects for offsets in interlaboratory calibrations, thus reducing systematic biases between reported δ234U values. Using this approach, we reassess (a) the value of modern seawater δ234U, and (b) the evolution of seawater δ234U over the last deglaciation. Modern coral δ234U values (145.0 ± 1.5‰) agree with previous measurements of seawater and modern corals only once the data have been normalized. Additionally, fossil corals in the surface ocean display δ234Ui values that are ∼5-7‰ lower during the last glacial maximum regardless of site, taxon, or diagenetic setting. We conclude that physical weathering of U-bearing minerals exposed during ice sheet retreat drives the increase in δ234U observed in the oceans, a mechanism that is consistent with the interpretation of the seawater Pb-isotope signal over the same timescale.

Atmospheric lead deposition to Okefenokee Swamp, Georgia, USA

USGS Publications Warehouse

Jackson, B.P.; Winger, P.V.; Lasier, P.J.

2004-01-01

'Capsule:' Coal combustion emissions appear to be a major source of Pb in the Okefenokee wetland. Contamination of the environment from atmospheric deposition during the twentieth century is pervasive even in areas ostensibly considered pristine or remote from point sources. In this study, Pb concentrations in a Pb-210-dated peat core collected from the Okefenokee Swamp, GA were used to assess historical contaminant input via atmospheric deposition. Lead isotope ratios were determined by dynamic reaction cell ICP-MS (DRC-ICP-MS). Increases in Pb concentration occurred in the late nineteenth century and a marked rise in Pb concentrations pre-dated the widespread use of leaded gasoline within the US. The Pb-206/Pb-207 ratios of 1.19 during this period were consistent with coal combustion emissions. A later increase in Pb concentration, concurrent with a trend toward more radiogenic Pb-206/Pb-207 ratios in gasoline is consistent with an increased input of Pb from leaded gasoline emissions. However, it appears that coal combustion emissions remain a major source of Pb to the Okefenokee.

Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U-Pb zircon geochronology

NASA Astrophysics Data System (ADS)

Rioux, Matthew; Garber, Joshua; Bauer, Ann; Bowring, Samuel; Searle, Michael; Kelemen, Peter; Hacker, Bradley

2016-10-01

The Semail (Oman-United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist- to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15-20 km). We have used high-precision U-Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12-95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (±0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicates that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

210Pb as a tool for establishing sediment chronologies: examples of potentials and limitations of conventional dating models.

PubMed

Kirchner, Gerald

2011-05-01

For aquatic sediments, the use of (210)Pb originating from the decay of atmospheric (222)Rn is a well-established methodology to estimate sediment ages and sedimentation rates. Traditionally, the measurement of (210)Pb in soils and sediments involved laborious and time-consuming radiochemical separation procedures. Due to the recent development of advanced planar ('n-type') semi-conductors with high efficiencies in the low-energy range which enable the gamma-spectrometric analysis of the 46.5 keV decay line of (210)Pb, sediment dating using this radionuclide has gained renewed interest. In this contribution, potentials and limitations of the (210)Pb methodology and of the models used for estimating sediment ages and sedimentation rates are discussed and illustrated by examples of freshwater and marine sediments. Comparison with the use of (137)Cs shows that the information which may be gained by these two tracers is complementary. As a consequence, both radionuclides should be used in combination for dating of recent sediments. It is shown that for various sedimentation regimes additional information from other sources (e.g. sediment lithology) may be needed to establish a reliable chronology. A strategy for sediment dating using (210)Pb is recommended. Copyright © 2010 Elsevier Ltd. All rights reserved.

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

USGS Publications Warehouse

Tatsumoto, M.; Unruh, D.M.; Desborough, G.A.

1976-01-01

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/204Pb vs 204Pb/204Pb plot. The slope of the best fit line is 0.6188 ?? 0.0016, yielding an isochron age of 4553 ?? 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ?? 25 m.y. and 107 ?? 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970). The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb. ?? 1976.

Dating the Gaofan and Hutuo Groups - Targets to investigate the Paleoproterozoic Great Oxidation Event in North China

NASA Astrophysics Data System (ADS)

Peng, Peng; Feng, Lianjun; Sun, Fengbo; Yang, Shuyan; Su, Xiangdong; Zhang, Zhiyue; Wang, Chong

2017-05-01

There are several sedimentary units in North China that are proposed to be associated with the Paleoproterozoic Great Oxidation Event (GOE) and/or subsequent events; however, few of them have been precisely dated. In this study, deposition age of the greenschist facies Gaofan and Hutuo Groups is determined. Zircon grains liberated from a tuff layer (metamorphosed to sericite-quartz schist) in the upper part of the Mohe Formation (the second of the three formations of the Gaofan Group) yield a weighted average 207Pb/206Pb age of 2186 ± 8 Ma (n = 7, MSWD = 1.3), representing time of deposition. This age and the detrital zircon U-Pb ages of the basal feldspar quartzite (meta-siltstone), as well as the initial deposition age of the unconformably overlying Hutuo Group, confine the deposition age of the Gaofan Group to 2350-2150 Ma. This result negates the Gaofan Group as one subgroup of the 2560-2510 Ma Wutai greenstone belt. Zircons from the Banlaoyao mafic sill (meta-diabase) that intruded the Dongye Subgroup of the Hutuo Group yield an upper intercept U-Pb age of 2057 ± 25 Ma (n = 14, MSWD = 1.3), representing time of crystallization. Considering the age of the basalt in the first formation of the Doucun Subgroup and the tuff in the first formation of the Dongye Subgroup, the deposition age of the Doucun and Dongye Subgroups of the Hutuo Group is confined to 2150-2090 Ma and 2090-2060 Ma, respectively. These age brackets, as well as the available carbon and nitrogen isotope data indicate that the Zhangxianbu Formation of the Gaofan Group possibly recorded the GOE; whereas the Mohe-Yaokouqian Formations of the Gaofan Group and the Doucun-Dongye Subgroups of the Hutuo Group recorded the subsequent Lomagundi-Jatuli Event (LJE). However, the Lomagundi-Jatuli carbon excursions are hardly distinguishable from the Gaofan Group and the Doucun Subgroup (Hutuo Group) as both units consist of little inorganic carbon but terrestrial clastic turbidites.

210Pb sediment dating in coastal transition zones: tropical saltmarshes.

NASA Astrophysics Data System (ADS)

Ruiz-Fernandez, A. C.; Sanchez-Cabeza, J. A.; Carnero-Bravo, V.; Perez-Bernal, L. H.

2016-12-01

Sea level rise (SLR) is one of the climate change effects expected to have the largest impact on coastal environments. SLR rates are not uniform around the planet and, therefore, local and regional data and trends are needed for proper adaptation plans. As long term monitoring stations of SLR are very scarce in most of the world, SLR trends obtained from 210Pb-dated sediment cores from tropical saltmarshes have become a practical alternative to obtain SLR trends within the past century, under the assumption that these ecosystems accrete at a similar rate to SLR. However, tropical saltmarshes are challenging environments for 210Pb dating: they are regularly dry, intermittently covered by seawater only during the highest tides, and sedimentary records often reflect the transition between terrestrial and marine environments (e.g. changes in grain size distribution, organic matter content and elemental composition) with all these factors contributing for atypical 210Pb depth profiles. In addition, 137Cs, the chronostratigraphic marker most commonly used to corroborate 210Pb dating, fails to be preserved in the sedimentary record in tropical areas, owing to its solubility in marine waters, if at all detectable. The present study describes the challenges and proposed solutions for 210Pb dating saltmarsh sediment cores from two saltmarsh areas (southern Gulf of California and Yucatan Peninsula) including the use of plutonium isotopes for corroboration purposes. Acknowledgements: projects CONACYT CB2010/153492 and PDCPN201301/214349; UNAM PAPIIT-IN203313 and the PRODEP network "Aquatic contamination: levels and effects" (year 3).

High Pb/Ce reservoir in depleted, altered mantle peridotites

NASA Astrophysics Data System (ADS)

Godard, M.; Kelemen, P.; Hart, S.; Jackson, M.; Hanghoj, K.

2005-12-01

We find consistent, high Pb/Ce in ICP-MS data for residual peridotites from the Mid-Atlantic Ridge (MAR, from ODP Leg 209), mid-ocean ridges (MOR) worldwide [1], Oman, Josephine and Trinity ophiolites, and the Jurassic Talkeetna arc. (MAR and Oman data from Montpellier; Josephine, Trinity and Talkeetna from WSU; some Pb concentrations checked by ID at WHOI). These samples have average Pb/Ce 10x primitive mantle (PM), with only 3 of 180 samples < PM. REE patterns and Ce concentration < PM in 165 of 180 samples are consistent with depletion via melt extraction, plus some magmatic refertilization. High Pb (average 3x PM, median 0.5x PM), could be due to (a) retention of Pb in residual sulfide, (b) addition of Pb in sulfide and plagioclase during `impregnation' by crystallizing melt, and/or (c) addition of Pb in sulfide and carbonate during alteration. Pb/Ce is correlated negatively with Ce concentration, suggesting a role for (a). Pb concentration is strongly correlated with Th and Nb. These elements are considered immobile during hydrothermal alteration, their correlations with Pb are positive, and Pb is > PM in many samples, all suggesting a complementary role for (b) and a limited role for (c). All samples except Talkeetna have Th/Pb < PM. All samples except some MOR peridotites also have U/Pb < PM. DRILLED MAR peridotites show U/Pb > PM in shallow, oxidized samples and < PM in downhole, reduced samples. Thus, high U/Pb in DREDGED MOR peridotites [1] is attributed to seafloor weathering. Given that oxidized weathering only extends tens of meters below the seafloor, we infer that most MOR peridotites have Th/Pb and U/Pb < PM. If they form with Pb isotope ratios similar to MORB, these rocks will evolve to values less radiogenic than the geochron. The effect of subduction modification on Th/Pb and U/Pb is unclear. For example, if elevated Pb is common in unaltered residual peridotites, subduction modification is likely to be minor. The size of the high Pb/Ce, low Th/Pb and U/Pb reservoir represented by these rocks depends on the reason for elevated Pb. We discuss three possibilities as outlined above. (a) Pb enrichment is most marked in highly depleted residues, abundant in the upper 30 km of the oceanic mantle. (b) Crystallization of igneous sulfide and plagioclase from cooling melt migrating along peridotite grain boundaries may be common in the upper 20 km in plates formed at slow spreading ridges. (c) Hydrothermal alteration of shallow mantle peridotite at slow spreading ridges might extend to 10 km. Based on these estimates, over geologic time tens of percent of mantle Pb could be sequestered in such a reservoir. This offers a potential solution to the "first lead paradox". [1] Niu, J. Petrol. 2004

The role of trace element partitioning between garnet, zircon and orthopyroxene on the interpretation of zircon U-Pb ages: an example from high-grade basement in Calabria (Southern Italy)

NASA Astrophysics Data System (ADS)

Fornelli, A.; Langone, A.; Micheletti, F.; Pascazio, A.; Piccarreta, G.

2014-03-01

The recognition of the coeval growth of zircon, orthopyroxene and garnet domains formed during the same metamorphic cycle has been attempted with detailed microanalyses coupled with textural analyses. A coronitic garnet-bearing granulite from the lower crust of Calabria has been considered. U-Pb zircon data and zircon, garnet and orthopyroxene chemistries, at different textural sites, on a thin section of the considered granulite have been used to test possible equilibrium and better constrain the geological significance of the U-Pb ages related to zircon separates from other rocks of the same structural level. The garnet is very rich in REE and is characterised by a decrease in HREE from core to outer core and an increase in the margin. Zircons show core-overgrowth structures showing different chemistries, likely reflecting episodic metamorphic new growth. Zircon grains in matrix, corona around garnet and within the inner rim of garnet, are decidedly poorer in HREE up to Ho than garnet interior. Orthopyroxene in matrix and corona is homogeneously poor in REE. Thus, the outer core of garnet and the analysed zircon grains grew or equilibrated in a REE depleted system due to the former growth of garnet core. Zircon ages ranging from 357 to 333 Ma have been determined in the matrix, whereas ages 327-320 Ma and around 300 Ma have been determined, respectively, on cores and overgrowths of zircons from matrix, corona and inner rim of garnet. The calculated DREEzrn/grt and DREEopx/grt are largely different from the equilibrium values of literature due to strong depletion up to Ho in zircon and orthopyroxene with respect to garnet. On the other hand, the literature data show large variability. In the case study, (1) the D zrn/grt values define positive and linear trends from Gd to Lu as many examples from literature do and the values from Er to Lu approach the experimental results at about 900 °C in the combination zircon dated from 339 to 305 Ma with garnet outer core, and (2) D opx/grt values define positive trends reaching values considered as suggestive of equilibrium from Er to Lu only with respect to the outer core of garnet. The presence of a zircon core dated 320 Ma in the inner rim of garnet suggests that it, as well as those dated at 325-320 Ma in the other textural sites and, probably, those dated at 339-336 Ma showing depletion of HREE, grew after the garnet core, which sequestered a lot of HREE and earlier than the HREE rich margin of garnet. The quite uniform REE contents in orthopyroxene from matrix and corona and the low and uniform contents of HREE in the zircon overgrowths dated at about 300 Ma allow to think that homogenisation occurred during or after the corona formation around this age. The domains dated around 325-320 Ma would approximate the stages of decompression, whereas the metamorphic peak probably occurred earlier than 339 Ma.

Race and Physical Attractiveness as Criteria for White Subjects' Dating Choices

ERIC Educational Resources Information Center

Allen, Bem P.

1976-01-01

Experiments involving "desirability for a date" ratings of black and white stimulus persons who varied in attractiveness indicated white male and female subjects gave appreciable weight to race and attractiveness, but females gave race more weight than attractiveness, while attractiveness was given more weight than race by males. (Author)

U-Th-Pb and Rb-Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow Valley

USGS Publications Warehouse

Nunes, P.D.; Tatsumoto, M.; Unruh, D.M.

1974-01-01

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using UThPb and RbSr systematics. A RbSr internal isochron age of 3.89 ?? 0.08 b.y. with an initial 87Sr/86Sr of 0.69926 ?? 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a UPb internal isochron of 3.8 ?? 0.2 b.y. and an initial 206Pb/207Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset RbSr and UPb mineral systems of older rocks. Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and 238U/204Pb values which are higher than those of group B. Group A rocks have typical 232Th/238U ratios of ??? 3.85, whereas-group B rocks have unusually high Th/U values of ??? 4.1. Whole-rock UPb and PbPb ages are nearly concordant. Two events appear to be reflected in these data - one at ??? 4.4 b.y. and one at ??? 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ??? 4.4 b.y. and ??? 4.5 b.y. may, like the younger ??? 4.0 b.y. ages, be related to basin excavation events. ?? 1974.

An Upper Paleozoic bio-chronostratigraphic scheme for the western margin of Gondwana

NASA Astrophysics Data System (ADS)

Césari, Silvia N.; Limarino, Carlos O.; Gulbranson, Erik L.

2011-05-01

The Carboniferous and Permian fossiliferous sequences of the central-western Argentina contain abundant plant remains, palynomorphs and invertebrates. They include a continuous record of large distribution in the Paganzo, Rio Blanco, Calingasta-Uspallata and San Rafael Basins. The most recent biostratigraphic schemes recognize a floristic succession represented by the biozones: Archaeosigillaria-Frenguellia (AF Biozone), Frenguellia eximia-Nothorhacopteris kellaybelenensis-Cordaicarpus cesarii (FNC Biozone), Nothorhacopteris-Botrychiopsis- Ginkgophyllum (NBG Biozone), Interval Biozone and Gangamopteris Biozone. The associated palynological record is represented by the biozones: Reticulatisporites magnidictyus-Verrucosisporites quasigobbetti (MQ Biozone), Raistrickia densa-Convolutispora muriornata (DM Biozone), Pakhapites fusus-Vittatina subsaccata (FS Biozone), and Lueckisporites-Weylandites (LW Biozone). The precise age of the Upper Paleozoic western Gondwanan biozones has been under discussion and remains controversial to date in some regions. The main issue hampering an integrated comparison of the Gondwanan biozones was its imprecise chronostratigraphic framework. However, new studies in some Argentinian stratigraphic sections bearing floras and faunas have yielded several radiometric ages. From these 206Pb/ 238U zircon datings it is possible to determine the chronostratigraphic range of many fossiliferous assemblages in this sector of Gondwana. In this way, the AF and MQ Biozones are restricted to the Late Mississippian and they would be not younger than 335 Ma according to radiometric ages. 206Pb/ 238U ages suggest that the NBG, DMa and DMb Biozones characterize the Late Serpukhovian glacial deposits and persisted up to the Late Bashkirian. Beds containing the Interval and DMc Biozones have yielded 206Pb/ 238U ages of 312.82 ± 0.11 Ma and 310.71 ± 0.1 Ma which would indicate that both zones characterize the Moscovian. The remains of Gangamopteris Biozone found in the Paganzo Basin overlie basalt levels ranging between 308 ± 6 and 293 ± 6 Ma. Therefore, the incoming of the first glossopterids was closely associated to the Carboniferous-Permian boundary in this part of Gondwana. The data presented in this paper are used for establishing comparisons with other Gondwanan biozones, constrained by absolute ages.

Geochemistry, zircon U-Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

NASA Astrophysics Data System (ADS)

Zeng, Renyu; Lai, Jianqing; Mao, Xiancheng; Li, Bin; Ju, Peijiao; Tao, Shilong

2016-05-01

Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U-Pb-Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072 °C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845 °C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U-Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4 ± 3.7 Ma and 361.7 ± 4.6 Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (-11 ± 0.5 to -9 ± 0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from -8 ± 1.0 to +10 ± 0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444 Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence after a period of extensional post-collisional intraplate evolution, strong compressive forces affected the area.

Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)

NASA Astrophysics Data System (ADS)

Fréville, Kévin; Trap, Pierre; Faure, Michel; Melleton, Jérémie; Li, Xian-Hua; Lin, Wei; Blein, Olivier; Bruguier, Olivier; Poujol, Marc

2018-02-01

A structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature-moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E-W- to NE-SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE-SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny.

Do Jack Hills Detrital Zircons Contain Records of the Early Geodynamo?

NASA Astrophysics Data System (ADS)

Weiss, B. P.; Maloof, A. C.; Tailby, N. D.; Ramezani, J.; Fu, R. R.; Glenn, D. R.; Kehayias, P.; Walsworth, R. L.; Hanus, V.; Trail, D.; Watson, E. B.; Harrison, T. M.; Bowring, S. A.; Kirschvink, J. L.; Swanson-Hysell, N.; Coe, R. S.; Einsle, J. F.; Harrison, R. J.

2015-12-01

It is unknown when Earth's dynamo magnetic field originated. With crystallization ages ranging from 3.0-4.38 Ga, detrital zircon crystals found in the Jack Hills of Western Australia might preserve a record of the missing first billion years of Earth's magnetic field history. Recently, Tarduno et al. (2015) argued that magnetization in Jack Hills zircons provides evidence for a substantial geomagnetic field dating back to their U/Pb formation ages (3.3 and 4.2 Ga). However, the identification of such ancient field records requires establishing that the zircons have avoided remagnetization since their formation. At a minimum, it should be demonstrated that they have not been remagnetized since being deposited at ~3.0 Ga. To establish the timing and intensity of the metamorphic and alteration events experienced by the zircon, we conducted 12 paleomagnetic field tests in combination with U-Pb geochronology on their host rocks (see Weiss et al. 2015, EPSL). Our data show that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely from emplacement of the Warakurna large igneous province at 1.1 Ga (see Figure). Even if some Jack Hills zircons do record a pre-depositional magnetization, they still could have been remagnetized sometime during the 1.4 Gy between their crystallization and their deposition. First, the temperatures capable of remagnetizing magnetite inclusions are well below those that could reset a U-Pb date or result in significant discordance. Therefore, thermal events capable of completely remagnetizing Jack Hills zircons could be undetected by the techniques reported by Tarduno at al. (2015). Second, the zircons' magnetization might be dominated by secondary ferromagnetic inclusions or contamination. To address the latter possibility, we are conducting electron microscopy, x-ray tomography, and magnetic field mapping on the zircons. Our initial quantum diamond magnetometry high-resolution (<20 um) imaging has found that the magnetization of many zircons resides in grain exteriors, suggesting a dominantly secondary origin. We conclude that there is currently no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons. We are continuing to search for such magnetic records.

Lu-Hf Garnet Geochronology Reveals the Tectonic History of Precambrian Rocks in the Southern Rocky Mountains

NASA Astrophysics Data System (ADS)

Aronoff, R.; Andronicos, C.; Vervoort, J. D.; Hunter, R. A.

2014-12-01

Lu-Hf garnet dating of Proterozoic rocks of the southwestern United States provides constraints on the timing and geographic extent of metamorphism associated with the Yavapai, Mazatzal, and newly recognized Picuris orogenies. Prior work focusing on U-Pb dating of plutons and Ar geochronology has left the timing of prograde metamorphism ambiguous, particularly in northern New Mexico and southern Colorado. Because the Lu-Hf system dates the onset of garnet growth, it can constrain the timing of the prograde P-T path. Garnet schist samples from central and northern New Mexico exhibit garnet growth restricted to the time period between ~1460 and 1400 Ma. In the Picuris and Manzano mountains, the oldest Lu-Hf garnet ages predate the U-Pb ages of ~1.4 Ga plutons located near the dated samples. This implies that garnet growth, and therefore the onset of amphibolite facies metamorphism, cannot be driven by contact metamorphism, as has been previously inferred. Garnet-bearing samples from the Needle and Wet Mountains in southern Colorado display a range of garnet ages between ~1750 and 1470 Ma. A garnet gneiss from the Needle Mountains in southwestern Colorado yields an age of 1748 Ma, which is consistent with the Yavapai orogeny. This Lu-Hf garnet age has not been reset by contact metamorphism associated with the emplacement of the ~1.4 Ga Eolus batholith. Anatectic garnet in an orthogneiss from the northern Wet Mountains yields an age of 1601 Ma and is interpreted to date partial melting at the close of the Mazatzal orogeny. A 1476 Ma garnet age from the aureole of the 1440 Ma Oak Creek pluton is interpreted to date upper amphibolite facies metamorphism. The age distribution of these samples shows that rocks in Colorado underwent a complex, poly-metamorphic history, while rocks in New Mexico underwent a single progressive metamorphic event. This contrast implies that the boundary between rocks deformed and metamorphosed during the ~1800-1600 Ma Yavapai and Mazatzal orogenies and those only deformed and metamorphosed during the ~1460-1400 Ma Picuris orogeny lies in northern New Mexico, along what has previously been considered the Mazatzal front. By using Lu-Hf geochronology to directly date a rock-forming mineral, we are better able to reconstruct the tectonic history of this region.

The Earth's Thorium and Uranium Abundance and Distribution

NASA Astrophysics Data System (ADS)

McDonough, W. F.; Guo, M.; Wipperfurth, S. A.

2017-12-01

The abundance and distribution of thorium (Th) and uranium (U) and its Th/U value have been assessed for the bulk silicate Earth (BSE), core, modern mantle and continental crust. These heat producing elements power the Earth's engine and are recorders of atmospheric oxidation and biologically mediated processes. More than 50,000 measured Th/U values (Kappa) and a couple of thousand Kappa-Pb values, the time-integrated Pb isotopic values from the decay of Th and U, are evaluated for the BSE, continental crust (CC) and modern mantle (MM), with the latter represented by mid-ocean ridge basalts (MORB) and ocean island basalt (OIB). The Kappa-Pb values for these complementary enriched and depleted domains of the BSE (i.e., CC_Kappa-Pb = 4.1 +/- 0.2 and MM_Kappa-Pb = 3.8 +/- 0.1, respectively) narrowly bracket the solar system initial (SSi_Kappa-Pb = 3.88 +/- 0.02) with an uncertainty of +/-5%) and demonstrate that negligible Th/U fractionation accompanied accretion, core formation, and crust - mantle differentiation. Experimental studies find marked differences in the partitioning of U and Th during core formation and thus, the BSE_Kappa-Pb = of 3.9 +/- 0.2 dictates that Th and U were excluded from the core. The <4% differences between the CC_Kappa-Pb and MM_Kappa-Pb reveals that U6+ recycling back into the mantle has either been a relatively recent process or that its recycling following atmospheric oxygenation at 2.4 Ga was limited and evolved slowly with time. Recent data from geoneutrino flux measurements at KamLAND observes a Th/U of 4.1 (+5.5, -3.3); although these uncertainties are large, future experiments, with annual count rates that are 10 to 40 times greater than that at KamLAND, will provide greater statistics, a critical measure of the planetary Th/U ratio, and an assessment of the assumption of chondritic ratio for the Earth.

Assessing Chronology and Mantle Evolution In-Situ with CODEX

NASA Astrophysics Data System (ADS)

Anderson, F. S.; Levine, J.; Whitaker, T.

2017-12-01

Understanding lunar bombardment history is crucial to understanding the dynamic evolution of the Moon. Using an instrument called CODEX (Chemistry, Organics, and Dating Experiment) intended for in-situ dating [1-5], we have obtained Pb-Pb dates for Martian meteorites Zagami and Northwest Africa (NWA) 7034, and lunar meteorites Miller Range 05035, LaPaz Icefield 02205, and NWA 032. In conjunction with our previous Rb-Sr success, these measurments demonstrate the potential for in-situ measurements of the Moon. Some of these Pb dates are consistent with young age estimates, however, many are consistent with previous anomalously old Pb measurements for lunar and SNC meteorites. Proposed explanations for this paradox include terrestrial Pb contamination, that the SNC's are actually ancient and reset in Rb/Sr, that there are multiple isotopic reservoirs sampled by the impact process, or that multiple reservoirs are sampled during the volcanic emplacement. In the future, we plan to use CODEX to test these hypotheses by making measurements on outcrops in-situ on Mars or the Moon, avoiding terrestrial or impact mixing. If in-situ Rb-Sr and Pb-Pb measurements are not concordant, then we are likely constraining the common Pb signature, and hence mantle evolution, of the Moon or Mars. Alternatively, we are likely obtaining a robust age estimate. References: [1] F. S. Anderson et al. LPSC 1246, 2 (2017); [2] F. S. Anderson et al. LPSC 2957, 2 (2017); [3] S. Beck et al., LPSC, 3001, 2 (2017); [4] T. J. Whitaker et al. LPSC 2328, 2 (2017); [5] F. S. Anderson et al. RCMS 29, 191 (2015);

U-Th-Pb geochronology of the Massabesic Gneiss and the granite near Milford, South-Central New Hampshire: New evidence for avalonian basement and taconic and alleghenian disturbances in Eastern New England

USGS Publications Warehouse

Aleinikoff, J.N.; Zartman, R.E.; Lyons, J.B.

1979-01-01

U-Th-Pb systematics for zircon and monazite from Massabesic Gneiss (paragneiss and orthogneiss) and the granite near Milford, New Hampshire, were determined. Zircon morphology suggests that the paragneiss may be volcaniclastic (igneous) in origin, and thus the age data probably record the date (minimum of 646 m.y.) at which the rock was extruded. A two-stage lead-loss model is proposed to explain the present array of data points on a concordia diagram. Orthogneiss ages range only narrowly and are clustered around 475 m.y. Data for the granite of Milford, New Hampshire, are scattered, but may be interpreted in terms of inheritance and modern lead loss, yielding a crystallization age of 275 m.y. This is the only known occurrence of Avalonian-type basement in New Hampshire and as such provides evidence for the location of the paleo-Africa-paleo- North America suture. The geochronology also further documents the occurrence of disturbances during the Ordovician and Permian. ?? 1979 Springer-Verlag.

Late Vendian Complexes in the Structure of Metamorphic Basement of the Fore Range Zone, Greater Caucasus

NASA Astrophysics Data System (ADS)

Kamzolkin, V. A.; Latyshev, A. V.; Vidyapin, Yu. P.; Somin, M. L.; Smul'skaya, A. I.; Ivanov, S. D.

2018-05-01

The paper presents new data on the composition, age, and relationships (with host and overlying deposits) of intrusive rocks in the basement of the Fore Range zone (Greater Caucasus), in the Malaya Laba River Basin. The evolutionary features of intrusive units located within the Blyb metamorphic complex are described. It is shown for the first time that the lower levels of this complex are, in a structural sense, outcrops of the Late Vendian basement. The basement is composed of the Balkan Formation and a massif of quartz metadiorites that intrudes it; for the rocks of this massif, ages ranging from 549 ± 7.4 to 574.1 ± 6.7 Ma are obtained for three U-Pb datings by the SHRIMP-II method. The Herzyinan magmatic event is represented by a group of granodiorite intrusions penetrating the Blyb complex on a series of faults extending along its boundary with the Main Range zone. The obtained estimate for the U-Pb age of one of the intrusions (319 ± 3.8 Ma) corresponds to the end of the Serpukhovian stage of the Early Carboniferous.

Absolute timing of sulfide and gold mineralization: A comparison of Re-Os molybdenite and Ar-Ar mica methods from the Tintina Gold Belt, Alaska

USGS Publications Warehouse

Selby, D.; Creaser, R.A.; Hart, C.J.R.; Rombach, C.S.; Thompson, J.F.H.; Smith, Moira T.; Bakke, A.A.; Goldfarb, R.J.

2002-01-01

New Re-Os molybdenite dates from two lode gold deposits of the Tintina Gold Belt, Alaska, provide direct timing constraints for sulfide and gold mineralization. At Fort Knox, the Re-Os molybdenite date is identical to the U-Pb zircon age for the host intrusion, supporting an intrusive-related origin for the deposit. However, 40Ar/39Ar dates from hydrothermal and igneous mica are considerably younger. At the Pogo deposit, Re-Os molybdenite dates are also much older than 40Ar/39Ar dates from hydrothermal mica, but dissimilar to the age of local granites. These age relationships indicate that the Re-Os molybdenite method records the timing of sulfide and gold mineralization, whereas much younger 40Ar/39Ar dates are affected by post-ore thermal events, slow cooling, and/or systemic analytical effects. The results of this study complement a growing body of evidence to indicate that the Re-Os chronometer in molybdenite can be an accurate and robust tool for establishing timing relations in ore systems.

Formation ages and evolution of Shergotty and its parent planet from U-Th-Pb systematics

NASA Technical Reports Server (NTRS)

Chen, J. H.; Wasserburg, G. J.

1986-01-01

The isotopic composition of Pb from Shergotty, Zagami, and EETA 79001 meteorites was determined for different phases. Using phases with low U-238/Pb-204 ratio, the initial leads of these meteorites were defined. Samples from all three meteorites were shown to have distinct initial leads, and, thus to have evolved from different reservoirs over most of solar system history in a low U-238/Pb-204 environment. It follows that the parent planet of the shergottites has a high Pb-204 concentration relative to U and must also be high in other volatiles. The possibility of the Martian origin of the SNC-type meteorites is discussed.

Dating of sediments from four Swiss prealpine lakes with (210)Pb determined by gamma-spectrometry: progress and problems.

PubMed

Putyrskaya, V; Klemt, E; Röllin, S; Astner, M; Sahli, H

2015-07-01

In this paper the most important problems in dating lake sediments with unsupported (210)Pb are summarized and the progress in gamma-spectrometry of the unsupported (210)Pb is discussed. The main topics of these studies concern sediment samples preparation for gamma-spectrometry, measurement techniques and data analysis, as well as understanding of accumulation and sedimentation processes in lakes. The vertical distributions of artificial ((137)Cs, (241)Am, (239)Pu) and natural radionuclides ((40)K, (210,214)Pb, (214)Bi) as well as stable trace elements (Fe, Mn, Pb) in sediment cores from four Swiss lakes were used as examples for the interpretation, inter-comparison and validation of depth-age relations established by three (210)Pb-based models (CF-CSR, CRS and SIT). The identification of turbidite layers and the influence of the turbidity flows on the accuracy of sediment dating is demonstrated. Time-dependent mass sedimentation rates in lakes Brienz, Thun, Biel and Lucerne are discussed and compared with published data. Copyright © 2015 Elsevier Ltd. All rights reserved.

A brief review of 210Pb sediment dating models and uncertainties in a world of global change

NASA Astrophysics Data System (ADS)

Sanchez-Cabeza, J. A.; Ruiz-Fernandez, A. C.

2016-12-01

Irrespective of the model names used, assumptions and (usually forgotten) uncertainties, the fact is that 210Pb sediment dating is an increasingly relevant tool in our world of global change. 210Pb dating results are needed to assess historical trends of sea level rise, quantify blue carbon fluxes and reconstruct environmental records of biogeochemical proxies for diverse processes in the aquatic ecosystems (such as ocean acidification, hypoxia and pollution). Although in the past 210Pb profiles departing from "ideal" decay trends were usually discarded, all profiles have useful information. In this work we review the principles and assumptions of the most common 210Pb dating models, and propose a logical formulation and classification of the models. 210Pb dating models provide two kinds of results: chronologies (i.e. age models) and accumulation rates. In many cases, the use of sediment and/or mass accumulation rates (SAR and MAR) is needed to assess environmental fluxes or, simply, to describe changes, such as catchment erosion or saltmarsh accretion. Although uncertainty quadratic propagation is a well-known technique, it requires that all variables are fully independent and requires demanding mathematical expressions which might lead to wrong results. We present here a Monte Carlo method that makes calculation easier and, likely, error-free. Not unexpectedly, the most important uncertainty sources are measurement uncertainties, which impose limitations on common techniques such as gamma spectrometry. 210Pb chronology does not cover all anthropogenic impacts, such as those caused by ancient civilizations, so radiocarbon also plays an important role in this kind of work. We also conceptually revise the limitations of both techniques and encourage scientists to link both dating techniques with a symmetrically open mind. Acknowledgements: projects CONACYT PDCPN2013-01/214349 and CB2010/153492, UNAM PAPIIT-IN203313, PRODEP network "Aquatic contamination: levels and effects" (year 3).

New U-Pb zircon ages and the duration and division of Devonian time

USGS Publications Warehouse

Tucker, R.D.; Bradley, D.C.; Ver Straeten, C.A.; Harris, A.G.; Ebert, J.R.; McCutcheon, S.R.

1998-01-01

Newly determined U-Pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the Devonian time-scale. They are: 1) 417.6 ?? 1.0 Ma for an ash within the conodont zone of Icriodus woschmidti/I. w. hesperius Lochkovian); 2) 408.3 ?? 1.9 Ma for an ash of early Emsian age correlated with the conodont zones of Po. dehiscens--Lower Po. inversus; 3) 391.4 ?? 1.8 Ma for an ash within the Po. c. costatus Zone and probably within the upper half of the zone (Eifelian); and 4) 381.1 ?? 1.3 Ma for an ash within the range of the Frasnian conodont Palmatolepis punctata (Pa. punctata Zone to Upper Pa. hassi Zone). U-Pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites-lepidophyta spore zone, are dated at 363.8 ?? 2.2 Ma and 363 ?? 2.2 Ma and 363.4 ?? 1.8 Ma, respectively, suggesting an age of ~363 Ma for a level within the late Famennian Pa. g. expansa Zone. These data, together with other published zircon ages, suggest that the base and top of the Devonian lie close to 418 Ma and 362 Ma, respectively, thus lengthening the period of ~20% over current estimates. We suggest that the duration of the Middle Devonian (Eifelian and Givitian) is rather brief, perhaps no longer than 11.5 Myr (394 Ma-382.5 Ma), and that the Emsian and Famennian are the longest stages in the period with estimated durations of ~15.5 Myr and 14.5 Myr, respectively.

Is 137Cs Dating Becoming Obsolete in North America?

NASA Astrophysics Data System (ADS)

Drexler, J. Z.; Fuller, C.; Salas, A.

2016-12-01

Dating of wetland sediments and peat is routinely carried out using 137Cs and 210Pb analysis. Unlike 210Pb, 137Cs is an anthropogenic radionuclide with a history of fallout from nuclear weapons testing. 137Cs is used as a single time marker; its peak is coincident with the height of atmospheric nuclear testing in 1963/4. During its use in the 1970s-90s, 137Cs peaks were usually highly distinct in wetland sediments (e.g., see 137Cs peaks from Louisiana marshes in Feijtel et al., 1988). This enabled its use as a check for dates assigned to a profile by 210Pb and other methods. However, recently, the efficacy of 137Cs dating in North America has deteriorated. In this presentation, we will provide specific examples of 137Cs as well as 210Pb dating in wetland sediments/peats we collected between 2005 and 2015 in Maine, California, Virginia, North Carolina, South Carolina, and Washington. Two main reasons exist for this decline. First, 137Cs activities in our recent cores are 30-40 % of the original activities in 1963/4 due to decay of the original 137Cs in situ (half-life = 30.17 years) and no major new sources. This manifests in lower signal to noise ratio, with some peaks barely recognizable above the noise. Second, 137Cs peaks are much less distinct due to 137Cs migration through time independent of substrate (or sediment) particles. Migration of peaks has resulted in estimated accretion rates being systematically lower or higher than those derived from 210Pb dating. These issues with 137Cs dating have important implications because 137Cs is used with 210Pb dating or even alone to determine rates of recent wetland carbon accumulation. Such rates are required to enter wetland restoration projects into carbon markets and to document IPCC mandated reductions in carbon pollution. Our analysis shows that, although dating by 137Cs alone has always been highly tenuous, now it is especially contraindicated and should be disallowed for the purposes of carbon accounting.

U-Th-Pb and Rb-Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow valley

NASA Technical Reports Server (NTRS)

Nunes, P. D.; Tatsumoto, M.; Unruh, D. M.

1974-01-01

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using U-Th-Pb and Rb-Sr systematics. A Rb-Sr internal isochron age of 3.89 plus or minus 0.08 b.y. with an initial Sr-87/Sr-86 of 0.69926 plus or minus 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a U-Pb internal isochron of 3.8 plus or minus 0.2 b.y. and an initial Pb-206/Pb-207 of 0.69. These internal isochron ages are interpreted as reflecting metamorphic events, probably related to impacts, which reset Rb-Sr and U-Pb mineral systems of older rocks.

The Mohawkian Chronostratigraphic Problem: building a reliable timescale by combining biostratigraphy, chemostratigraphy, and tephrochronology

NASA Astrophysics Data System (ADS)

Sell, B. K.; Sadler, P.; Leslie, S.; Mitchell, C.; Samson, S. D.

2011-12-01

The abundant exposures of Mohawkian (late Sandbian to early Katian) sedimentary rocks in eastern North America have been well-studied for insights into the Taconic orogeny and potential petroleum sources. Considerable information has been published toward establishing the sequence stratigraphic architecture, biozones for conodonts, graptolites and chitinozoans, chemostratigraphic correlations and a tephrochronologic framework. And yet, correlation remains difficult. Problems arise from complex sedimentary facies changes across the Laurentian margin and associated provincalism of the faunas. The difficulties are exacerbated by some imprecise usage of bentonite names, the short time spans of key stratigraphic sections, and a paucity of sections with muliple kinds of information. Also, linking so many taxon range end, ash-fall, and stable isotope excursion events into a coherent stratigraphic sequence is a daunting numerical problem. It falls into the notorious "NP-Complete" category because the number of possible solutions grows so fast as the number of events increases. "Simulated annealing" is one of the algorithms developed for such problems. We adopt it to solve the stratigraphic sequencing problem as a constrained optimization (CONOP). Nevertheless, to realize the full potential, more bentonite charactization and dating is needed in sections with detailed range charts for fossil species. CONOP works best with the individual taxon ranges, not the derived biozone boundaries. We examine the potential resolving power of CONOP in the context of a re-evaluation of bentonite correlations and newly acquired CA-TIMS U-Pb zircon dates from sections with rich biostratigraphic data. In particular we use 206Pb/238U zircon dates from two bentonites in the Womble Shale at the Katian Global Stratotype Section and Point (452.8 ± 0.2 and 453.5 ± 0.3 Ma, weighted mean with 2σ internal error) to compare various correlations with other dated bentonites in eastern North America. Preliminary dates for five long-range (>1000 km) bentonite correlations made on the basis of single-crystal apatite trace-element analyses (Mg, Cl, Mn, Fe, Ce, Y) help resolve previous conflicts among various biostratgraphic data, but raise new questions about the interpretation of stratigraphic sequences and chemostratigraphic data.

Leaching behavior of U, Mn, Sr, and Pb from different particle-size fractions of uranium mill tailings.

PubMed

Liu, Bo; Peng, Tongjiang; Sun, Hongjuan

2017-06-01

Pollution by the release of heavy metals from tailings constitutes a potential threat to the environment. To characterize the processes governing the release of Mn, Sr, Pb, and U from the uranium mill tailings, a dynamic leaching test was applied for different size of uranium mill tailings samples. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) were performed to determine the content of Mn, Sr, Pb, and U in the leachates. The release of mobile Mn, Sr, Pb, and U fraction was slow, being faster in the initial stage and then attained a near steady-state condition. The experimental results demonstrate that the release of Mn, Sr, Pb, and U from uranium mill tailings with different size fractions is controlled by a variety of mechanisms. Surface wash-off is the release mechanism for Mn. The main release mechanism of Sr and Pb is the dissolution in the initial leaching stage. For U, a mixed process of wash-off and diffusion is the controlling mechanism.

Time and duration of metamorphism and exhumation of the central Rhodopian core complex, Bulgaria

NASA Astrophysics Data System (ADS)

Ovtcharova, M.; von Quadt, A.; Peytcheva, I.; Neubauer, F.; Heinrich, C. A.; Kaiser, M.

2003-04-01

The evolution of central Rhodopian dome (Bulgaria) is interpreted in terms of an extensional collapse of thickened crust (Ivanov at al., 2000). U-Pb isotope dating (single Zr and Mnz), Rb-Sr (W.R., Bt and Ap) and Ar-Ar (on Bt) were carried out on different rocks from the central Rhodope, Bulgaria, to constrain the timing and duration of the metamorphism and exhumation of the core complex. The beginning of extensional stage is marked by intrusion of earliest non-penetratively deformed granite bodies at 53Ma (U-Pb on single Zr and Mnz). The late Alpine extensional evolution of the massif is marked by a detachment system connected with exhumation of the migmatites in the core part of the dome (lower plate). U-Pb analyses on Mnz and Zr from mesosome and discordant leucosome yield a Variscan protolith age of the gneiss (311 Ma) and Eocene age (37Ma) of crystallization of the newly formed anatectic melt that corresponds with the peak of the Alpine metamorphic event (P 4.5-6kbar and T 720-750^oC; Georgieva et al., 2002). Rb-Sr mineral system of the weakly deformed gneisses from lower plate of the core complex gives evidence for a cooling age of 34.5±0.34Ma. This result is confirmed by Ar-Ar on Bt from the same rock: 35.5±0.4Ma. Ar-Ar data on biotite from gneisses of the upper plate yield an age of 34.9±0.6Ma. The same age is reflected by an Rb-Sr isochron (W.R., Bt and Ap) of 35.22±0.35Ma. The post-collisional extension was followed by graben depressions filled with sediments of Eocene-Oligocene age and active volcanism and ore mineralization (Zn-Pb and Cu-Pb-Zn ore deposits). Connected with the most intensively "stretched" sections of the extensional system is emplacement of rhyolitic dikes at 32.8±0.41Ma (U-Pb on single Zr, Xe). The available data constrain narrow time bracket between timing of high-grade metamorphism event (37Ma, >600^oC), cooling (35Ma, 300ºC) of the core complex and volcanic activity (32Ma) that corresponds with rapid exhumation tectonic regime. References: Ivanov, Z., D. Dimov, S. Sarov. 2000.ABCD-GEODE workshop, Borovets,Guide to excursion (B), 6-17. Georgieva, M., Z. Cherneva, K. Kolcheva, S. Sarov, J. Gerdjikov, E. Voinova 2002. Scientific meeting of the Geological Institute BAS, (in press).

U-Pb systematics in iron meteorites - Uniformity of primordial lead

NASA Astrophysics Data System (ADS)

Gopel, C.; Manhes, G.; Allegre, C. J.

1985-08-01

Pb isotopic compositions and U-Pb abundances were determined in the metal phase of six iron meteorites: Canyon Diablo IA, Toluca IA, Odessa IA, Youndegin IA, Deport IA, and Mundrabilla An. Prior to complete dissolution, samples were subjected to a series of leachings and partial dissolutions. Isotopic compositions and abundances of the etched Pb indicate a contamination by terrestrial Pb which is attributable to previous cutting of the meteorite. Pb isotopic compositions measured in the decontaminated samples are identical within 0.2 percent and essentially confirm the primordial Pb value defined by Tatsumoto et al. (1973). These data invalidate more radiogenic Pb isotopic compositions published for iron meteorites, which are the result of terrestrial Pb contamination introduced mainly by analytical procedure. The results of this study support the idea of a solar nebula which was isotopically homogeneous for Pb 4.55 Ga ago. The new upper limit for U-abundance in iron meteorites, 0.001 ppb, is in agreement with its expected thermodynamic solubility in the metal phase.

Tracking the multiple-stage exhumation history and magmatic-hydrothermal events of the West Junggar region, NW China: Evidence from 40Ar/39Ar and (U-Th)/He thermochronology

NASA Astrophysics Data System (ADS)

Yin, Jiyuan; Chen, Wen; Xiao, Wenjiao; Long, Xiaoping; Tao, Ni; Liu, Li-Ping; Yuan, Chao; Sun, Min

2018-06-01

To decipher cooling events in the West Junggar region, biotite and K-feldspar 40Ar/39Ar, and zircon and apatite (U-Th)/He isotopic analyses of intrusive rocks were carried out. Previous U-Pb data showed that intrusive bodies in the Baogutu area were emplaced at 315-310 Ma. U-Pb and zircon (U-Th)/He dating results (313-241 Ma) suggest that a magmatic-hydrothermal event lasted for 72 Ma in the Baogutu area of the West Junggar region. Early-stage high temperature alteration (900-300 °C) lasted for 6-2 Ma and was followed by prolonged phyllic and argillic alteration lasting 67-63 Ma between 350 and 200 °C. Finally, slower cooling occurred between 200 and 70 °C, accompanied by post-mineralization uplift and erosion. In this study, three main episodes of relatively rapid cooling were distinguished in the West Junggar region, i.e. late Carboniferous-early Permian (307-277 Ma), middle Triassic (241-232 Ma) and early Cretaceous (145-120 Ma). The first rapid cooling during the late Carboniferous-early Permian was possibly associated with the release of magmatic heat. The middle Triassic and early Cretaceous cooling and exhumation are interpreted as a response to collision(s) between the Qiangtang and Kunlun-Qaidam or Lhasa blocks. The Cenozoic India-Eurasia collision, however, may have had little or no effect on modern tectonic reactivation of the West Junggar region.

Milk and Dairy Products Intake Is Associated with Low Levels of Lead (Pb) in Workers highly Exposed to the Metal.

PubMed

Gomes, Willian Robert; Devóz, Paula Pícoli; Araújo, Marília Ladeira; Batista, Bruno Lemos; Barbosa, Fernando; Barcelos, Gustavo Rafael Mazzaron

2017-07-01

Lead (Pb) is a toxic metal, frequently associated with occupational exposure, due to its widespread use in industry and several studies have shown high Pb levels in workers occupationally exposed to the metal. The aim of this study was to evaluate the influence of milk and dairy products (MDP) on Pb levels in blood (B-Pb), plasma (P-Pb), and urine (U-Pb), in workers from automotive battery industries in Brazil. The study included 237 male workers; information concerning diet and lifestyle were gathered through a questionnaire, and B-Pb, P-Pb, and U-Pb were determined by ICP-MS. Mean B-Pb, P-Pb, and U-Pb were 21 ± 12, 0.62 ± 0.73 μg/dL, and 39 ± 47 μg/g creatinine, respectively. Forty three percent of participants declared consuming ≤3 portions/week of MDP (classified as low-MDP intake), while 57% of individuals had >3portions/week of MDP (high-MDP intake). B-Pb and P-Pb were correlated with working time (r s  = 0.21; r s  = 0.20; p < 0.010). Multivariable linear regressions showed a significant influence of MDP intake on B-Pb (β = -0.10; p = 0.012) and P-Pb (β = -0.16; p < 0.010), while no significance was seen on U-Pb. Our results suggest that MDP consumption may modulate Pb levels in individuals highly exposed to the metal; these findings may be due to the Pb-Ca interactions, since the adverse effects of Pb are partially based on its interference with Ca metabolism and proper Ca supplementation may help to reduce the adverse health effects induced by Pb exposure.

Percolation of diagenetic fluids in the Archaean basement of the Franceville basin

NASA Astrophysics Data System (ADS)

Mouélé, Idalina Moubiya; Dudoignon, Patrick; Albani, Abderrazak El; Cuney, Michel; Boiron, Marie-Christine; Gauthier-Lafaye, François

2014-01-01

The Palaeoproterozoic Franceville basin, Gabon, is mainly known for its high-grade uranium deposits, which are the only ones known to act as natural nuclear fission reactors. Previous work in the Kiéné region investigated the nature of the fluids responsible for these natural nuclear reactors. The present work focuses on the top of the Archaean granitic basement, specifically, to identify and date the successive alteration events that affected this basement just below the unconformity separating it from the Palaeoproterozoic basin. Core from four drill holes crosscutting the basin-basement unconformity have been studied. Dating is based on U-Pb isotopic analyses performed on monazite. The origin of fluids is discussed from the study of fluid inclusion planes (FIP) in quartz from basement granitoids. From the deepest part of the drill holes to the unconformable boundary with the basin, propylitic alteration assemblages are progressively replaced by illite and locally by a phengite + Fe chlorite ± Fe oxide assemblage. Illitic alteration is particularly strong along the sediment-granitoid contact and is associated with quartz dissolution. It was followed by calcite and anhydrite precipitation as fracture fillings. U-Pb isotopic dating outlines three successive events: a 3.0-2.9-Ga primary magmatic event, a 2.6-Ga propylitic alteration and a late 1.9-Ga diagenetic event. Fluid inclusion microthermometry suggests the circulation of three types of fluids: (1) a Na-Ca-rich diagenetic brine, (2) a moderately saline (diagenetic + meteoric) fluid, and (3) a low-salinity fluid of probable meteoric origin. These fluids are similar to those previously identified within the overlying sedimentary rocks of the Franceville basin. Overall, the data collected in this study show that the Proterozoic-Archaean unconformity has operated as a major flow corridor for fluids circulation, around 1.9 Ga. highly saline diagenetic brines; hydrocarbon-rich fluids derived from organic matter-rich formations; a low-salinity fluid likely of meteoric origin migrating through the granitic basement; mineralizing fluids resulting from the mixing of fluids 1 and 3; high-temperature fluids resulting from the natural nuclear reactor environment (Mathieu et al., 2000). The present paper attempts to characterize the succession of alteration events that have affected the top of the basement below the Palaeoproterozoic sediment unconformity. Are these alterations related to early post-magmatic to hydrothermal events, to palaeoweathering, or to late infiltration of diagenetic brines from the overlying basin? Our study, carried out on drill core samples from Kiéné, is supported by petrographic investigation, new fluid inclusion data and U-Pb geochronology on monazite.

Chemical and biological methods to evaluate the availability of heavy metals in soils of the Siena urban area (Italy).

PubMed

Nannoni, Francesco; Protano, Giuseppe

2016-10-15

A biogeochemistry field study was conducted in the Siena urban area (Italy) with the main objective of establishing the relationship between available amounts of heavy metals in soil assessed by a chemical method (soil fractionation) and bioavailability assessed by a biological method (bioaccumulation in earthworm tissues). The total content of traffic-related (Cd, Cu, Pb, Sb, Zn) and geogenic (Co, Cr, Ni, U) heavy metals in uncontaminated and contaminated soils and their concentrations in soil fractions and earthworms were used for this purpose. The bioavailability of heavy metals assessed by earthworms did not always match the availability defined by soil fractionation. Earthworms were a good indicator to assess the bioavailability of Pb and Sb in soil, while due to physiological mechanisms of regulation and excretion, Cd, Cu and Zn tissue levels in these invertebrates gave misleading estimates of their bioavailable pool. No relationship was identified between chemical and biological availability for the geogenic heavy metals, characterized by a narrow range of total contents in soil. The study highlighted that chemical and biological methods should be combined to provide more complete information about heavy element bioavailability in soils. Copyright © 2016 Elsevier B.V. All rights reserved.

Formation and Geological Sequestration of Uranium Nanoparticles in Deep Granitic Aquifer

PubMed Central

Suzuki, Yohey; Mukai, Hiroki; Ishimura, Toyoho; Yokoyama, Takaomi D.; Sakata, Shuhei; Hirata, Takafumi; Iwatsuki, Teruki; Mizuno, Takashi

2016-01-01

The stimulation of bacterial activities that convert hexavalent uranium, U(VI), to tetravalent uranium, U(IV), appears to be feasible for cost-effective remediation of contaminated aquifers. However, U(VI) reduction typically results in the precipitation of U(IV) particles less than 5 nanometers in diameter, except for environmental conditions enriched with iron. Because these tiny particles are mobile and susceptible to oxidative dissolution after the termination of nutrient injection, in situ bioremediation remains to be impractical. Here we show that U(IV) nanoparticles of coffinite (U(SiO4)1−x(OH)4x) formed in fracture-filling calcium carbonate in a granitic aquifer. In situ U-Pb isotope dating demonstrates that U(IV) nanoparticles have been sequestered in the calcium carbonate for at least 1 million years. As the microbiologically induced precipitation of calcium carbonate in aquifer systems worldwide is extremely common, we anticipate simultaneous stimulation of microbial activities for precipitation reactions of calcium carbonate and U(IV) nanoparticles, which leads to long-term sequestration of uranium and other radionuclides in contaminated aquifers and deep geological repositories. PMID:26948389

Formation and Geological Sequestration of Uranium Nanoparticles in Deep Granitic Aquifer.

PubMed

Suzuki, Yohey; Mukai, Hiroki; Ishimura, Toyoho; Yokoyama, Takaomi D; Sakata, Shuhei; Hirata, Takafumi; Iwatsuki, Teruki; Mizuno, Takashi

2016-03-07

The stimulation of bacterial activities that convert hexavalent uranium, U(VI), to tetravalent uranium, U(IV), appears to be feasible for cost-effective remediation of contaminated aquifers. However, U(VI) reduction typically results in the precipitation of U(IV) particles less than 5 nanometers in diameter, except for environmental conditions enriched with iron. Because these tiny particles are mobile and susceptible to oxidative dissolution after the termination of nutrient injection, in situ bioremediation remains to be impractical. Here we show that U(IV) nanoparticles of coffinite (U(SiO4)1-x(OH)4x) formed in fracture-filling calcium carbonate in a granitic aquifer. In situ U-Pb isotope dating demonstrates that U(IV) nanoparticles have been sequestered in the calcium carbonate for at least 1 million years. As the microbiologically induced precipitation of calcium carbonate in aquifer systems worldwide is extremely common, we anticipate simultaneous stimulation of microbial activities for precipitation reactions of calcium carbonate and U(IV) nanoparticles, which leads to long-term sequestration of uranium and other radionuclides in contaminated aquifers and deep geological repositories.

In situ U-Th-Pb ages of the Miaoya carbonatite complex in the South Qinling orogenic belt, central China

NASA Astrophysics Data System (ADS)

Ying, Yuancan; Chen, Wei; Lu, Jue; Jiang, Shao-Yong; Yang, Yueheng

2017-10-01

The Miaoya carbonatite complex in the South Qinling orogenic belt hosts one of the largest rare earth element (REE)-Nb deposits in China that is composed of carbonatite and syenite. The emplacement age of the complex and the geochronological relationship between the carbonatite and syenite have long been debated. In this study, in situ U-Th-Pb ages have been obtained for the constituent minerals zircon, monazite and columbite from carbonatite and syenite of the Miaoya complex, together with their chemical and isotopic compositions. In situ trace element compositions for zircon from carbonatite and syenite are highly variable. The zircon displays slightly heavy REE (HREE)-enriched chondrite-normalized patterns with no Eu anomaly and various light REE (LREE) contents. In situ Th-Pb dating for zircon from the Miaoya complex by laser ablation ICP-MS yields ages of 442.6 ± 4.0 Ma (n = 53) for syenite and 426.5 ± 8.0 Ma (n = 23) for carbonatite. Monazite from carbonatite and syenite shows similar chondrite-normalized REE patterns and yields a consistent Th-Pb age of 240 Ma. Based on petrographic and chemical composition, columbite from the carbonatite can be identified into two groups. The columbite dispersed within carbonatite is characterized by slightly LREE-enriched chondrite-normalized REE patterns, whereas columbite associated with apatite is characterized by LREE-depleted trends. Columbite has been further determined to have a weighted mean 206Pb/238U age of 232.8 ± 4.5 Ma (n = 9) using LA-ICP-MS. Detailed geochronological and chemical investigations suggest that there were two major episodes of magmatic/metasomatic activities in the formational history of the Miaoya carbonatite complex. The early alkaline magmatism emplaced in the Silurian was related to the opening of the Mianlue Ocean, whereas the late metasomatism or hydrothermal overprint occurred during the Triassic South Qinling orogeny. The latter serves as the major ore formation period for both REE (e.g., monazite) and Nb (e.g., columbite).

U-Pb systematics in coexisting zircon, rutile and titanite from granophyres in the Archean Stillwater Complex: metamictization and the fate of radiogenic Pb

NASA Astrophysics Data System (ADS)

Friedman, R. M.; Wall, C. J.; Scoates, J. S.; Meurer, W. P.

2009-12-01

Self-irradiation of zircon causes structural damage (metamictization) that can result in the loss of radiogenic Pb during interaction with aqueous solutions. To evaluate this behavior in metamict zircon, and in other U-bearing accessory phases like titanite and rutile, we are examining the U-Pb systematics of granophyric rocks from the ca. 2.7 Ga Stillwater layered intrusion, Montana. Four samples were studied in detail, including a pegmatitic ksp-qtz core to a gabbroic pegmatoid in the Lower Banded Series (N1), an alaskite and an amphibole-rich reaction zone between the alaskite and anorthosite (AN1) in the Middle Banded Series, and an amphibole-bearing granophyre from the Upper Banded Series (GN3). Except in the pegmatite, zircon is variably metamict with amorphous zones characterized by distinctive Ca-enrichment. Single zircon grains were analyzed by ID-TIMS following annealing and chemical abrasion, and multi-grain (n=4-5) fractions of titanite and rutile were analyzed by conventional ID-TIMS; the UBC 233-235U-205Pb isotopic tracer is calibrated against mixed U-Pb gravimetric reference solutions made available through the EarthTime initiative. The U-Pb systematics are coherent only for the pegmatite yielding both a Concordia age of 2709.60 ± 0.80 Ma (2σ, including tracer calibration, decay-constant errors not included) for low-U zircon (76-237 ppm) and concordant titanite results with 207Pb/206Pb ages from 2701-2710 Ma. The results for high-U zircon (up to 1438 ppm) for the other three samples are strongly discordant (9-43%, 85-89%, 28-71%, respectively) with a wide range of 207Pb/206Pb ages (2583-2647 Ma, 2210-2357 Ma, 2345-2499 Ma). Given the extreme incompatibility of Pb2+ in zircon and the highly metamict state of zircon in these granophyres, we are investigating the extent to which radiogenic lead is selectively removed during the chemical abrasion and annealing process from step-wise leaching experiments and image analysis (CL, SEM). In contrast, titanite and rutile analyses yield highly variable results that are grouped based on degree of discordance: (1) minor to moderate discordance (0-20%) with titanite ages at ca. 2.7 Ga and rutile ages from 2.6-2.7 Ga, (2) strong discordance (34-78%) with variable and young ages defined by fractions with relatively high-U contents (60-774 ppm), and (3) strong reverse discordance (3-21%), especially titanite from the reaction zone, which nonetheless gives 207Pb/206Pb ages of ca. 2705 Ma. We speculate that reverse discordance may be related to elemental redistribution in the granophyres during hydrothermal alteration of the co-existing highly metamict zircon, and other U-bearing phases, and subsequent incorporation of Pb into co-existing titanite and rutile.

Effect of cadmium, chromium, lead and metal mixtures on survival and growth of juveniles of the scallop Argopecten ventricosus (Sowerby II, 1842).

PubMed

Sobrino-Figueroa, Alma S; Cáceres-Martínez, Carlos; Botello, Alfonso V; Nunez-Nogueira, G

2007-08-01

The effects of Cd, Cr, Pb and their mixtures on the growth and sensitivity of the scallop Argopecten ventricosus were analyzed in this study. Cadmium showed to be more toxic metal to juveniles (96 hour median lethal concentration (LC(50)) = 0.396 mg Cd/L), followed by lead (LC(50) = 0.830 mg Pb/L) and chromium (LC(50) = 3.430 mg Cr/L). Cadmium toxicity was 8 times higher than chromium and 2 times than lead. The most toxic combination was Cd + Cr + Pb. (LC(50) = 0.302 mg/L). Based on toxic units analyses (T.U.), a synergistic effect was observed for Cr + Pb and Cd + Cr + Pb. (T.U. = 0.374; T.U. = 0.403), and antagonic effects for Cd + Cr and Cd + Pb (T.U. = 1.26; T.U. = 1.43) respectively. The level of effect (from high to low) on the growth of A. ventricosus juveniles was: Cd > Cd + Cr + Pb > Cr > Pb. The EC(50) (metal concentration where a reduction of 50% growing rate is observed) obtained were: Cd = 0.018 mg/L, Cd + Cr + Pb = 0.104 mg/L, Cr = 0.51 mg/L and Pb = 4.21 mg/L. These results suggest that A. ventricosus juveniles are more sensitive to these metals in comparison to other juveniles from other bivalve species (e.g., A. irradians, Mytillus edulis, Crassostrea virginica).

U-Pb ID-TIMS zircon ages of TTG gneisses of the Aravalli Craton of India

NASA Astrophysics Data System (ADS)

Chauhan, Hiredya; Saikia, Ashima; Kaulina, Tatiana; Bayanova, Tamara; Ahmad, Talat

2015-04-01

The crystalline basement of the Aravalli Craton is a heterogeneous assemblage dominated by granitic gneisses and granites with sporadic occurrences of amphibolites and dismembered sedimentary enclaves (Upadhyaya et al., 1992). This assemblage is known to have experienced multiple deformation and metamorphic events followed by emplacement of voluminous granites and basaltic dykes. Based on Sm-Nd whole rock data on the basement Mewar orthogneisses of Jhamarkotra region (Gopalan et al., 1990) and Pb/Pb ages of zircon from Gingla Granites which intrudes the basement (Wiedenbeck et al., 1996), it has been inferred that the whole magmatic episode leading to the formation of the basement spanned from 3300 to 2400 Ma and that the Aravalli cratonic block had broadly stabilized by 2500 Ma on which the younger Aravalli and Delhi Supergroup unconformably deposited. However, no comprehensive age data on the basement gneisses from the study area spanning the entire magmatic episode is available. This work attempts to provide a time frame work for evolution of the basement gneisses of the Aravalli Craton. We present here U-Pb zircon ages from the Precambrian basement TTG gneisses of the Aravalli Craton of north western India. Pb and U were measured on multicollector Finnigan-MAT 262 mass spectrometer. The temperatures of measurements were 1300°C for Pb and 1500°C for U. Pb isotope ratios were corrected for mass fractionation with a factor of 0.10% per amu, based on repeat analyses of the standard NBS SRM 982. The U analyses were corrected for mass fractionation with a factor of 0.003% per amu, based on repeat analyses of the NBS U 500 standard. Reproducibility of the U-Pb ratios was determined from the repeated analysis of standard zircon IGFM-87 (Ukraine) and taken as 0.5% for 207Pb/235U and 206Pb/238U ratios, respectively, at 95% confidence level. All calculations were done using the programs PBDAT and ISOPLOT (Ludwig 1991, 2008). Four zircon fractions corresponding to four zircon types from UD-16 sample yield a U-Pb discordant age of 2680±30 Ma. Two zircon fractions from UD-17 sample show discordant 207Pb/206Pb ages of 2506 and 2577 Ma. Zircons in our samples have moderate to high U contents (180-770 ppm) with low Th/U ratios (0.2-0.5) in the sample UD-16, characteristic for magmatic zircons from TTG rocks. Thus the obtained age of 2680±30 Ma is interpreted as an age of magmatic crystallization of tonalites. Gopalan, K. et al., (1990): Precambrian Res., 48, 287-297. Ludwig, K.R. (1991): PBDAT program. US. Geol. Surv. Open-file report 88-542, 38 p. Ludwig, K. R. (2008): Isoplot/Ex, version 3.6, Berkeley Geochronology Center, Special Publication no. 4. Upadhyaya, R. et al., (1992): Current Sci., 62(2): 87-92. Wiedenbeck, M. et al., (1996): Chem Geol. 129: 325-340.

I-Xe Dating: Comparison of I-Xe and Pb-Pb Ages of Richardton Chondrules and Separated Mineral Phases

NASA Technical Reports Server (NTRS)

Pravdivtseva, O. V.; Amelin, Y.; Hohenberg, C. M.; Meshik, A. P.

2002-01-01

I-Xe and Pb-Pb ages of individual Richardton chondrules and different mineral phases were compared in order to test the absolute I-Xe age normalization. Additional information is contained in the original extended abstract.

Environmental evolution records reflected by radionuclides in the sediment of coastal wetlands: A case study in the Yellow River Estuary wetland.

PubMed

Wang, Qidong; Song, Jinming; Li, Xuegang; Yuan, Huamao; Li, Ning; Cao, Lei

2016-10-01

Vertical profiles of environmental radionuclides ( 210 Pb, 137 Cs, 238 U, 232 Th, 226 Ra and 4 0 K) in a sediment core (Y1) of the Yellow River Estuary wetland were investigated to assess whether environmental evolutions in the coastal wetland could be recorded by the distributions of radionuclides. Based on 210 Pb and 137 Cs dating, the average sedimentation rate of core Y1 was estimated to be 1.0 cm y -1 . Vertical distributions of natural radionuclides ( 238 U, 232 Th, 226 Ra and 40 K) changed dramatically, reflecting great changes in sediment input. Concentrations of 238 U, 232 Th, 226 Ra and 40 K all had significant positive relationships with organic matter and clay content, but their distributions were determined by different factors. Factor analysis showed that 238 U was determined by the river sediment input while 226 Ra was mainly affected by the seawater erosion. Environmental changes such as river channel migrations and sediment discharge variations could always cause changes in the concentrations of radionuclides. High concentrations of 238 U and 226 Ra were consistent with high accretion rate. Frequent seawater intrusion decreased the concentration of 226 Ra significantly. The value of 238 U/ 226 Ra tended to be higher when the sedimentation rate was low and tide intrusion was frequent. In summary, environmental evolutions in the estuary coastal wetland could be recorded by the vertical profiles of natural radionuclides. Copyright © 2016 Elsevier Ltd. All rights reserved.

Lead concentration in plasma as a biomarker of exposure and risk, and modification of toxicity by δ-aminolevulinic acid dehydratase gene polymorphism.

PubMed

Tian, Liting; Zheng, Guang; Sommar, Johan Nilsson; Liang, Yihuai; Lundh, Tomas; Broberg, Karin; Lei, Lijian; Guo, Weijun; Li, Yulan; Tan, Mingguang; Skerfving, Staffan; Jin, Taiyi; Bergdahl, Ingvar A

2013-08-14

Blood lead concentration (B-Pb), the main biomarker of lead exposure and risk, is curvi-linearily related to exposure. We assessed plasma lead (P-Pb) as a marker for both lead exposure and toxic effects. We examined claims that δ-aminolevulinic acid dehydratase genotype (ALAD) can modify lead toxicity. In 290 lead-exposed and 91 unexposed Chinese workers, we determined P-Pb, B-Pb, urinary lead (U-Pb), ALAD polymorphism (rs1800435, ALAD1/2; TaqMan assay), and also toxic effects on heme synthesis (blood zinc protoporphyrin and hemoglobin, urinary δ-aminolevulic acid), on the kidneys (urinary albumin, β2-microglobulin and N-acetyl-β-d-glucosaminidase) and on the peripheral nervous system (sensory and motor conduction velocities). In exposed workers, median P-Pb was 4.10 (range 0.35-27)μg/L, B-Pb 401 (110-950)μg/L, and U-Pb 188 (22-590)μg/g creatinine. P-Pb had a higher ratio between exposed and unexposed workers (median 39, range 18-110) than B-Pb (19, 15-36; p<0.001) and U-Pb (28, 15-36; p<0.001). All three biomarkers were associated with all toxic effects (P-Pb: rS=-0.10 to 0.79; B-Pb: rS=-0.08 to 0.75; all p<0.05). In the exposed workers, B-Pb and U-Pb were significantly higher (p=0.04) in ALAD2 carriers (7% in the exposed population) than in ALAD1 homozygotes. P-Pb values were similar; ALAD1 homozygotes suffered higher kidney toxicity at the same P-Pb. (i) P-Pb has advantages over B-Pb as a biomarker of high Pb exposure, but it was not significantly better as an index of risk of toxicity. (ii) The ALAD genotype modifies toxicokinetics and toxicodynamics. Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.

Analysis of U.S. soil lead (Pb) studies from 1970 to 2012.

PubMed

Datko-Williams, Laura; Wilkie, Adrien; Richmond-Bryant, Jennifer

2014-01-15

Although lead (Pb) emissions to the air have substantially decreased in the United States since the phase-out of leaded gasoline by 1995, amounts of lead in some soils remain elevated. Lead concentrations in residential and recreational soils are of concern because health effects have been associated with Pb exposure. Elevated soil Pb is especially harmful to young children due to their higher likelihood of soil ingestion. The purpose of this study is to create a comprehensive compilation of U.S. soil Pb data published from 1970 through 2012 as well as to analyze the collected data to reveal spatial and/or temporal soil Pb trends in the U.S. over the past 40 years. A total of 84 soil Pb studies across 62 U.S. cities were evaluated. Median soil Pb values from the studies were analyzed with respect to year of sampling, residential location type (e.g., urban, suburban), and population density. In aggregate, there was no statistically significant correlation between year and median soil Pb; however, within single cities, soil Pb generally declined over time. Our analysis shows that soil Pb quantities in city centers were generally highest and declined towards the suburbs and exurbs of the city. In addition, there was a statistically significant, positive relationship between median soil Pb and population density. In general, the trends examined here align with previously reported conclusions that soil Pb levels are higher in larger urban areas and Pb tends to remain in soil for long periods of time. © 2013.

The Middle Triassic insect radiation revealed by isotopic age and iconic fossils from NW China

NASA Astrophysics Data System (ADS)

Zheng, Daran; Chang, Su-Chin; Wang, He; Fang, Yan; Wang, Jun; Feng, Chongqing; Xie, Guwei; Jarzembowski, Edmund A.; Zhang, Haichun; Wang, Bo

2017-04-01

Following the end-Permian mass extinction, the Triassic represented an important period witnessing the recovery and radiation of marine and terrestrial ecosystems. Terrestrial plants and vertebrates have been widely investigated; however the insects, the most diverse organisms on earth, remain enigmatic due to the rarity of Early-Middle Triassic fossils. Here we report new fossils from a Ladinian deposit dated at 238-237 Ma and a Carnian deposit in northwestern China, including the earliest definite caddisfly cases (Trichoptera) and water boatmen (Hemiptera), diverse polyphagan beetles (Coleoptera) and scorpionflies (Mecoptera). Our findings suggest that the Holometabola, comprising the majority of modern-day insect species, experienced an extraordinary diversification in the Middle Triassic and was already been dominant in some Middle and Late Triassic insect faunas, after the extinction of several ecologically dominant, Paleozoic insect groups in the latest Permian and earliest Triassic. This turnover is perhaps related to notable episodes of extreme warming and drying, leading to the eventual demise of coal-swamp ecosystems, evidenced by floral turnover during this interval. The forest revival during the Middle Triassic probably stimulated the rapid radiation and evolution of insects including some key aquatic lineages which built new associations that persist to the present day. Our results provide not only new insights into the early evolution of insect diversity and ecology, but also robust evidence for the view that the Triassic is the "Dawn of the Modern World". Besides, LA-ICP-MS U-Pb dating initially gave a late Ladinian age for the Tongchuan entomnfauna after the results: 237.41 ± 0.91 Ma and 238 ± 0.97 Ma. The age is in agreement with that of the marine Ladinian-Carnian boundary, representing a novel age constraint for the terrestrial strata near this boundary. This age can provide a calibration for marine and terrestrial correlation near Ladinian-Carnian boundary, and also for the correlation of the contemporaneous biotas.

Magmatism and crustal extension: Constraining activation of the ductile shearing along the Gediz detachment, Menderes Massif (western Turkey)

NASA Astrophysics Data System (ADS)

Rossetti, Federico; Asti, Riccardo; Faccenna, Claudio; Gerdes, Axel; Lucci, Federico; Theye, Thomas

2017-06-01

The Menderes Massif of western Turkey is a key area to study feedback relationships between magma generation/emplacement and activation of extensional detachment tectonics. Here, we present new textural analysis and in situ U-(Th)-Pb titanite dating from selected samples collected in the transition from the undeformed to the mylonitized zones of the Salihli granodiorite at the footwall of the Neogene, ductile-to-brittle, top-to-the-NNE Gediz-Alaşheir (GDF) detachment fault. Ductile shearing was accompanied by the fluid-mediated sub-solidus transformation of the granodiorite to orthogneiss, which occurred at shallower crustal levels and temperatures compatible with the upper greenschist-to-amphibolite facies metamorphic conditions (530-580 °C and P < 2 GPa). The syn-tectonic metamorphic overgrowth of REE-poor titanite on pristine REE-rich igneous titanite offers the possibility to constrain the timing of magma crystallisation and solid-state shearing at the footwall of the Gediz detachment. The common Pb corrected 206Pb/238U (206Pb*/238U) ages and the REE re-distribution in titanite that spatially correlates with the Th/U zoning suggests that titanite predominantly preserve open-system ages during fluid-assisted syn-tectonic re-crystallisation in the transition from magma crystallization and emplacement (at 16-17 Ma) to the syn-tectonic, solid-state shearing (at 14-15 Ma). A minimum time lapse of ca. 1-2 Ma is then inferred between the crustal emplacement of the Salihli granodiorite and nucleation of the ductile extensional shearing along the Gediz detachment. The reconstruction of the cooling history of the Salihli granodiorite documents a punctuated evolution dominated by two episodes of rapid cooling, between 14 Ma and 12 Ma ( 100 °C/Ma) and between 3 and 2 Ma ( 105 °C/Ma). We relate the first episode to nucleation and development of post-emplacement of ductile shearing along the GDF and the second to brittle high-angle faulting, respectively. Our dataset suggests that in the Menderes Massif the activation of ductile extension was a consequence, rather than the cause, of magma emplacement in the extending crust.

Is the Macquarie Arc (Lachlan Orogen) An Exotic Terrane or Formed on the Gondwanan Margin? Reappraisal by SHRIMP U-Pb Dating of Volcano-Sedimentary Rocks

NASA Astrophysics Data System (ADS)

Buckman, S.; Zhang, Q.; Nutman, A. P.

2017-12-01

A fundamental question concerning the Ordovician Macquarie Arc rocks is did they form within the palaeo-Pacific Ocean and are entirely juvenile, or did they evolve on the periphery of Gondwana? This is a key issue to ongoing debates concerning the growth of the eastern Gondwanan margin throughout the Palaeozoic. This problem is complicated by the arc now occurs as several slices, in post-arc tectonic contact with the eastern Gondwanan Ordovician Adaminaby Group. The dispersal of the arc as tectonic slices means that the temporal correlation of lithologies across the extent of the arc's exposure needs to be verified via U-Pb zircon geochronology. Our zircon U-Pb geochronology reveals that samples with the oldest zircons of Palaeozoic volcanic origin do not contain any Gondwanan-sourced zircons (particularly Cambrian and Neoproteorzoic). These samples, particularly some ascribed to the Weemalla Formation and Mitchell Formation have unimodal zircon populations of 450.5 Ma and 479.8 Ma. On the other hand, some samples with somewhat younger volcanic populations of 415 Ma and 458 Ma contain some Gondwanan-sourced older detrital zircons as well. Some of these latter samples are derived from outcrops that have previously consigned to the Yarrimbah Formation, should be older than, or equivalent to, the samples with the unimodal volcanic zircon populations of 480 Ma. This shows clearly that the consignment of some Macquarie Arc units to particular formations needs to be revised. The geochemical and radiogenic isotopic characteristics of the Macquarie Arc indicate that it is dominated by products of an intra-oceanic island arc that developed contemporaneously but spatially separated from Adaminaby Group passive margin sedimentation along eastern Gondwana. However, because our new U-Pb zircon data reveals that only samples with the youngest volcanic zircons also contain Gondwanan zircons, it shows that before the death of the arc, it was proximal to the Gondwanan margin, leading to a mixed provenance. This model is being tested by zircon Hf isotopic analysis.

Sandstone provenance and U-Pb ages of detrital zircons from Permian-Triassic forearc sediments within the Sukhothai Arc, northern Thailand: Record of volcanic-arc evolution in response to Paleo-Tethys subduction

NASA Astrophysics Data System (ADS)

Hara, Hidetoshi; Kunii, Miyuki; Miyake, Yoshihiro; Hisada, Ken-ichiro; Kamata, Yoshihito; Ueno, Katsumi; Kon, Yoshiaki; Kurihara, Toshiyuki; Ueda, Hayato; Assavapatchara, San; Treerotchananon, Anuwat; Charoentitirat, Thasinee; Charusiri, Punya

2017-09-01

Provenance analysis and U-Pb dating of detrital zircons in Permian-Triassic forearc sediments from the Sukhothai Arc in northern Thailand clarify the evolution of a missing arc system associated with Paleo-Tethys subduction. The turbidite-dominant formations within the forearc sediments include the Permian Ngao Group (Kiu Lom, Pha Huat, and Huai Thak formations), the Early to earliest Late Triassic Lampang Group (Phra That and Hong Hoi formations), and the Late Triassic Song Group (Pha Daeng and Wang Chin formations). The sandstones are quartzose in the Pha Huat, Huai Thak, and Wang Chin formations, and lithic wacke in the Kiu Lom, Phra That, Hong Hoi and Pha Daeng formations. The quartzose sandstones contain abundant quartz, felsic volcanic and plutonic fragments, whereas the lithic sandstones contain mainly basaltic to felsic volcanic fragments. The youngest single-grain (YSG) zircon U-Pb age generally approximates the depositional age in the study area, but in the case of the limestone-dominant Pha Huat Formation the YSG age is clearly older. On the other hand, the youngest cluster U-Pb age (YC1σ) represents the peak of igneous activity in the source area. Geological evidence, geochemical signatures, and the YC1σ ages of the sandstones have allowed us to reconstruct the Sukhothai arc evolution. The initial Sukhothai Arc (Late Carboniferous-Early Permian) developed as a continental island arc. Subsequently, there was general magmatic quiescence with minor I-type granitic activity during the Middle to early Late Permian. In the latest Permian to early Late Triassic, the Sukhothai Arc developed in tandem with Early to Middle Triassic I-type granitic activity, Middle to Late Triassic volcanism, evolution of an accretionary complex, and an abundant supply of sediments from the volcanic rocks to the trench through a forearc basin. Subsequently, the Sukhothai Arc became quiescent as the Paleo-Tethys closed after the Late Triassic. In addition, parts of sediments of supposed Devonian-Carboniferous age within the Sukhothai Arc were revised as the Triassic Lampang Group, and the Early Cretaceous Khorat Group.

The origin of zircon and the significance of U-Pb ages in high-grade metamorphic rocks: a case study from the Variscan orogenic root (Vosges Mountains, NE France)

NASA Astrophysics Data System (ADS)

Skrzypek, E.; Štípská, P.; Cocherie, A.

2012-12-01

U-Pb zircon dating is combined with petrology, Zr-in-rutile thermometry and mineral equilibria modelling to discuss zircon petrogenesis and the age of metamorphism in three units of the Variscan Vosges Mountains (NE France). The monotonous gneiss unit shows results at 700-500 Ma, but no Variscan ages. The varied gneiss unit preserves ages between 600 and 460 Ma and a Variscan group at 340-335 Ma. Zircon analyses from the felsic granulite unit define a continuous array of ages between 500 and 340 Ma. In varied gneiss samples, zoned garnet includes kyanite and rutile and is surrounded by matrix sillimanite and cordierite. In a pseudosection, it points to peak conditions of ~16 kbar/850 °C followed by isothermal decompression to 8-10 kbar/820-860 °C. In felsic granulite samples, the assemblage K-feldspar-garnet-kyanite-Zr-rich rutile is replaced by sillimanite and Zr-poor rutile. Modelling these assemblages supports minimum conditions of ~13 kbar/925 °C, and a subsequent P-T decrease to 6.5-8.5 kbar/800-820 °C. The internal structure and chemistry of zircons, and modelling of zircon dissolution/growth along the inferred P-T paths are used to discuss the significance of the U-Pb ages. In the monotonous unit, inherited zircon ages of 700-500 Ma point to sedimentation during the Late Cambrian, while medium-grade metamorphism did not allow the formation of Variscan zircon domains. In both the varied gneiss and felsic granulite units, zircons with a blurred oscillatory-zoned pattern could reflect solid-state recrystallization of older grains during HT metamorphism, whereas zircons with a dark cathodoluminescence pattern are thought to derive from crystallization of an anatectic melt during cooling at middle pressure conditions. The present work proposes that U-Pb zircon ages of ca. 340 Ma probably reflect the end of a widespread HT metamorphic event at middle crustal level.

Uranium-lead systematics of low-Ti basaltic meteorite Dhofar 287A: Affinity to Apollo 15 green glasses

NASA Astrophysics Data System (ADS)

Terada, Kentaro; Sasaki, Yu; Anand, Mahesh; Sano, Yuji; Taylor, Lawrence A.; Horie, Kenji

2008-06-01

Dhofar 287 is a lunar meteorite found in Oman in 2001, which consists of a major portion (95%) of low-Ti mare basalt (Dho 287A) and a minor attached part (˜ 5%) of regolith breccia (Dho 287B). Here, we report the U-Pb systematics of Dho 287A using data collected with a Sensitive High Resolution Ion Microprobe (SHRIMP). In-situ analyses of five merrillite and three apatite grains, which are resistant to secondary petrologic events, resulted in a total Pb/U isochron age of 3.34 ± 0.20 Ga, in 238U/206Pb-207Pb/206Pb-204Pb/206Pb 3-D space (95% confidence level). The observed Pb-Pb isochron of these eight phosphates coupled with four plagioclase grains also yielded a 207Pb/206Pb age of 3.35 ± 0.13 Ga. This formation age, when considered as the crystallization age of Dho 287A, is similar to crystallization ages of Apollo 15 low-Ti olivine-normative basalts (ONB; 3.3 ± 0.1 Ga). However, the estimated μ-value (238U/204Pb ratio) of Dho 287A is ˜ 18, which is very different from the reported μ-values of ˜ 300 for mare basalts from the Apollo collections, including the Apollo 15 ONBs. These μ-values are still significantly lower than those of Apollo KREEP basalt (500 to 1000), although a possible assimilation with KREEP has been previously proposed for Dho 287A using geochemical criteria. Our U-Pb study of Dho 287A, instead, indicates a closer affinity to Apollo 15 green glasses (207Pb/206Pb age of 3.41 Ga with μ-value of 19 to 55), which are considered to be the most primitive products of lunar volcanism. Combining our U-Pb data with the previously reported Sm-Nd systematics (negative ɛNd) of Dho 287A clearly distinguishes this meteorite from those of the Yamato 793169 and Asuka 88175 group which have extremely low μ-value of 10-22, old crystallization ages of 3.9 Ga, and high positive ɛNd, suggesting that Dho 287A may be a representative of an entirely new group of mare basalt derived from previously unsampled source region on the Moon.

Adverse health effects of lead exposure on children and exploration to internal lead indicator.

PubMed

Wang, Q; Zhao, H H; Chen, J W; Gu, K D; Zhang, Y Z; Zhu, Y X; Zhou, Y K; Ye, L X

2009-11-15

Our research on adverse effects of lead exposures on physical and neurobehavioral health of children aged 6-12years in 4 villages, labeled as K, M, L, and X, in rural China, was reported in this article. Lead in blood (PbB), urine (PbU), hairs (PbH), and nails (PbN) were measured by graphite furnace atomic absorption spectrometry. Abbreviated Symptom Questionnaire of Conner's instruments and Revised Raven's Standard Progressive Matrices were applied to evaluate childhood attention deficit/hyperactivity disorders (ADHD) and intelligences. Geometric means (SD) of PbB, PbU, PbH and PbN concentrations were 71.2 microg/L (1.56), 11.7 microg/g (1.75), 12.5 microg/g (2.82), and 25.3 microg/g (2.79), respectively. 54 (17.0%) children had PbB levels of > or =100 microg/L. Boys, 6-10 years old, and living in village K were 2.11, 2.48, and 9.16 times, respectively, more likely to be poisoned by lead than girls, aged 11-12 years, and residing in X. 18 (5.7%) and 37 (11.7%) subjects had ADHD and mental retardations, respectively. Inverse relationships between intelligences and natural log transformed PbU and PbH levels were observed with respective odds ratios (95%CI) of 1.79 (1.00-3.22) and 1.46 (1.06-2.03) or 1.28 (1.04-1.58) and 1.73 (1.18-2.52) by binary or ordinal logistic regression modeling. ADHD prevalence was different by gender and age of subjects. PbU, PbH, and PbN related to PbB positively with respective correlation coefficients of 0.530, 0.477, and 0.181. Receiver operating characteristic (ROC) curves of the three measurements revealed areas under curves (AUCs) being 0.829, 0.758, and 0.687, respectively. In conclusion, children had moderate levels of lead exposures in this rural area. Intelligence declines were associated with internal lead levels among children. ROC analysis suggests PbU an internal lead indicator close to PbB.

Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

USGS Publications Warehouse

Aleinikoff, J.N.; Horton, J. Wright; Walter, M.

1996-01-01

Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12-29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38-52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ?? 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ?? 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ?? 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

U-Th-Pb systematics of selected samples from Apollo 17, Boulder 1, Station 2

USGS Publications Warehouse

Nunes, P.D.; Tatsumoto, M.

1975-01-01

Nine U-Th-Pb whole-rock analyses of selected brecciated materials from sample 72215 and one analysis of a pigeonite basalt clast from 72275 are presented. Both samples are from Boulder 1, Apollo 17. These data supplement previous Boulder 1 U-Th-Pb analyses of samples 72275 and 72255. U and Th concentrations indicate that most of the samples contain a moderate to large KREEP component. Samples containing the least KREEP are a noritic clast (72255,49; Civet Cat clast) and an anorthositic clast (72275,117). Evidence for the migration of Pb from Pb-rich matrix material into relatively Pb-poor clasts is presented for two clasts. Most of the Boulder 1 data define a linear trend that intersects concordia at ??? 3.9 and 4.4 b.y. when plotted on a U-Pb concordia diagram. The presence of one anorthositic clast distinctly off this trend indicates that a simple two-stage U-Pb evolution history is inadequate to explain all the data. Accordingly physical significance is only attached to the lower concordia intercept age of 3.9-4.0 b.y. The older concordia intercept age of ??? 4.4 b.y. is interpreted to reflect an averaging of events both older and younger than 4.4 b.y. The data suggest that significant differentiation and/or metamorphism occurred ??? 4.2 b.y. ago. The age of this event, however, is not accurately defined by these data. ?? 1975 D. Reidel Publishing Company, Dordrecht-Holland.

Cation vacancies in ferroelectric PbTiO3 and Pb(Zr,Ti)O3 : A positron annihilation lifetime spectroscopy study

NASA Astrophysics Data System (ADS)

Keeble, D. J.; Singh, S.; Mackie, R. A.; Morozov, M.; McGuire, S.; Damjanovic, D.

2007-10-01

Positron annihilation lifetime spectroscopy measurements identify A - and B -site cation vacancies in ferroelectric perovskite oxides (ABO3) . Crystal PbTiO3 and ceramic lead zirconium titanate (PZT) were studied and gave consistent values for the lifetime resulting from positron localization at lead vacancies VPb . Positron trapping to B -site vacancies was inferred in PZT. Temperature dependent studies showed that the defect specific trapping rate was higher for VB compared to VPb , consistent with the larger negative charge. Doping PZT with Fe increased the fraction positron trapping to VB compared to VPb -type defects.

Whole blood lead levels are associated with biomarkers of joint tissue metabolism in African American and white men and women: the Johnston County Osteoarthritis Project.

PubMed

Nelson, Amanda E; Chaudhary, Sanjay; Kraus, Virginia B; Fang, Fang; Chen, Jiu-Chiuan; Schwartz, Todd A; Shi, Xiaoyan A; Renner, Jordan B; Stabler, Thomas V; Helmick, Charles G; Caldwell, Kathleen; Poole, A Robin; Jordan, Joanne M

2011-11-01

To examine associations between biomarkers of joint tissue metabolism and whole blood lead (Pb), separately for men and women in an African American and Caucasian population, which may reflect an underlying pathology. Participants in the Johnston County Osteoarthritis Project Metals Exposure Sub-Study (329 men and 342 women) underwent assessment of whole blood Pb and biochemical biomarkers of joint tissue metabolism. Urinary cross-linked N telopeptide of type I collagen (uNTX-I) and C-telopeptide fragments of type II collagen (uCTX-II), serum cleavage neoepitope of type II collagen (C2C), serum type II procollagen synthesis C-propeptide (CPII), and serum hyaluronic acid (HA) were measured using commercially available kits; the ratio of [C2C:CPII] was calculated. Serum cartilage oligomeric matrix protein (COMP) was measured by an in-house assay. Multiple linear regression models were used to examine associations between continuous blood Pb and biomarker outcomes, adjusted for age, race, current smoking status, and body mass index. Results are reported as estimated change in biomarker level for a 5-unit change in Pb level. The median Pb level among men and women was 2.2 and 1.9μg/dL, respectively. Correlations were noted between Pb levels and the biomarkers uNTX-I, uCTX-II, and COMP in women, and between Pb and uCTX-II, COMP, CPII, and the ratio [C2C:CPII] in men. In adjusted models among women, a 5-unit increase in blood Pb level was associated with a 28% increase in uCTX-II and a 45% increase in uNTX-I levels (uCTX-II: 1.28 [95% CI: 1.04-1.58], uNTX-I: 1.45 [95% CI:1.21-1.74]). Among men, levels of Pb and COMP showed a borderline positive association (8% increase in COMP for a 5-unit change in Pb: 1.08 [95% CI: 1.00-1.18]); no other associations were significant after adjustment. Based upon known biomarker origins, the novel associations between blood Pb and biomarkers appear to be primarily reflective of relationships to bone and calcified cartilage turnover among women and cartilage metabolism among men, suggesting a potential gender-specific effect of Pb on joint tissue metabolism that may be relevant to osteoarthritis. Copyright © 2011 Elsevier Inc. All rights reserved.

Whole blood lead levels are associated with biomarkers of joint tissue metabolism in African American and White men and women: The Johnston County Osteoarthritis Project

PubMed Central

Nelson, Amanda E.; Chaudhary, Sanjay; Kraus, Virginia B.; Fang, Fang; Chen, Jiu-Chiuan; Schwartz, Todd A.; Shi, Xiaoyan A.; Renner, Jordan B.; Stabler, Thomas V.; Helmick, Charles G.; Caldwell, Kathleen; Poole, A. Robin; Jordan, Joanne M.

2011-01-01

Purpose To examine associations between biomarkers of joint tissue metabolism and whole blood lead (Pb), separately for men and women in an African American and Caucasian population, which may reflect an underlying pathology. Methods Participants in the Johnston County Osteoarthritis Project Metals Exposure Sub-study (329 men and 342 women) underwent assessment of whole blood Pb and biochemical biomarkers of joint tissue metabolism. Urinary cross-linked N telopeptide of type I collagen (uNTX-I) and C-telopeptide fragments of type II collagen (uCTX-II), and serum cleavage neoepitope of type II collagen (C2C), serum type II procollagen synthesis C-propeptide (CPII), and serum hyaluronic acid (HA) were measured using commercially available kits; the ratio of [C2C:CPII] was calculated. Serum cartilage oligomeric matrix protein (COMP) was measured by an in-house assay. Multiple linear regression models were used to examine associations between continuous blood Pb and biomarker outcomes, adjusted for age, race, current smoking status, and body mass index. Results are reported as estimated change in biomarker level for a 5-unit change in Pb level. Results The median Pb level among men and women was 2.2 and 1.9 µg/dL, respectively. Correlations were noted between Pb levels and the biomarkers uNTX-I, uCTX-II, and COMP in women, and between Pb and uCTX-II, COMP, CPII, and the ratio [C2C:CPII] in men. In adjusted models among women, a 5-unit increase in blood Pb level was associated with a 28% increase in uCTX-II and a 45% increase in uNTX-I levels (uCTX-II: 1.28 [95%CI: 1.04–1.58], uNTX-I: 1.45 [95%CI:1.21–1.74]). Among men, levels of Pb and COMP showed a borderline positive association (8% increase in COMP for a 5-unit change in Pb: 1.08 [95% CI: 1.00–1.18])); no other associations were significant after adjustment. Conclusions Based upon known biomarker origins, the novel associations between blood Pb and biomarkers appear to be primarily reflective of relationships to bone and calcified cartilage turnover among women and cartilage metabolism among men, suggesting a potential gender-specific effect of Pb on joint tissue metabolism that may be relevant to osteoarthritis. PMID:21839992

Mercury contamination history of an estuarine floodplain reconstructed from a 210Pb-dated sediment core (Berg River, South Africa).

PubMed

Kading, T J; Mason, R P; Leaner, J J

2009-01-01

Mercury deposition histories have been scarcely documented in the southern hemisphere. A sediment core was collected from the ecologically important estuarine floodplain of the Berg River (South Africa). We establish the concentration of Hg in this (210)Pb-dated sediment core at <50 ng g(-1) Hg(T) throughout the core, but with 1.3 ng g(-1) methylmercury in surface sediments. The (210)Pb dating of the core provides a first record of mercury deposition to the site and reveals the onset of enhanced mercury deposition in 1970. The ratio of methylmercury to total mercury is relatively high in these sediments when compared to other wetlands.

Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss

USGS Publications Warehouse

Mojzsis, Stephen J.; Cates, Nicole L.; Caro, Guillaume; Trail, Dustin; Abramov, Oleg; Guitreau, Martin; Blichert-Toft, Janne; Hopkins, Michelle D.; Bleeker, Wouter

2014-01-01

The oldest compiled U–Pb zircon ages for the Acasta Gneiss Complex in the Northwest Territories of Canada span about 4050–3850 Ma; yet older ca. 4200 Ma xenocrystic U–Pb zircon ages have also been reported for this terrane. The AGC expresses at least 25 km2 of outcrop exposure, but only a small subset of this has been documented in the detail required to investigate a complex history and resolve disputes over emplacement ages. To better understand this history, we combined new ion microprobe235,238U–207,206Pb zircon geochronology with whole-rock and zircon rare earth element compositions ([REE]zirc), Ti-in-zircon thermometry (Tixln) and 147Sm–143Nd geochronology for an individual subdivided ∼60 cm2 slab of Acasta banded gneiss comprising five separate lithologic components. Results were compared to other variably deformed granitoid-gneisses and plagioclase-hornblende rocks from elsewhere in the AGC. We show that different gneissic components carry distinct [Th/U]zirc vs. Tixln and [REE]zirc signatures correlative with different zircon U–Pb age populations and WR compositions, but not with 147Sm–143Nd isotope systematics. Modeled  [REE] from lattice-strain theory reconciles only the ca. 3920 Ma zircons with the oldest component that also preserves strong positive Eu∗ anomalies. Magmas which gave rise to the somewhat older (inherited) ca. 4020 Ma AGC zircon age population formed at ∼IW (iron–wüstite) to

Crustal evolution at mantle depths constrained from Pamir xenoliths

NASA Astrophysics Data System (ADS)

Kooijman, E.; Hacker, B. R.; Smit, M. A.; Kylander-Clark, A. R.; Ratschbacher, L.

2012-12-01

Lower crustal xenoliths erupted in the Pamir at ~11 Ma provide an exclusive opportunity to study the evolution of crust at mantle depths during a continent-continent collision. To investigate, and constrain the timing of, the petrologic processes that occurred during burial to the peak conditions (2.5-2.8 GPa, 1000-1100 °C; [1]), we performed chemical- and isotope analyses of accessory minerals in 10 xenoliths, ranging from eclogites to grt-ky-qtz granulites. In situ laser ablation split-stream ICPMS yielded 1) U-Pb ages, Ti concentrations and REE in zircon, 2) U/Th-Pb ages and REE in monazite, and 3) U-Pb ages and trace elements in rutile. In addition, garnet, and biotite and K-feldspar were dated using Lu-Hf and 40Ar/39Ar geochronology, respectively. Zircon and monazite U-(Th-)Pb ages are 101.9±1.8, 53.7±1.0, 39.1±0.8, 21.7±0.4, 18.2±0.5, 16.9±0.8, 15.1±0.3 (2σ) and 12.5-11.1 Ma; most samples showed several or all of these populations. The 53.7 Ma and older ages are xenocrystic or detrital. For younger ages, zircon and monazite in individual samples recorded different ages-although zircon in one rock and monazite in another can be the same age. The 39.1 Ma zircon and monazite mostly occur as inclusions in minerals of the garnet-bearing assemblage that represents the early, low-P stages of burial. Garnet Lu-Hf ages of 37.8±0.3 Ma support garnet growth at this time. Spinifex-like textures containing 21.7-11.1 Ma zircon and monazite record short-lived partial melting events during burial. Aligned kyanite near these patches indicates associated deformation. Zircons yielding ≤12.5 Ma exhibit increased Eu/Eu* and markedly decreased HREE concentrations, interpreted to record feldspar breakdown and omphacite growth during increasing pressure. Rutile U-Pb cooling ages are 10.8±0.3 Ma in all samples. This agrees with the weighted mean 40Ar/39Ar age of eight biotite, K-feldspar and whole rock separates of 11.00+0.16/-0.09 Ma. Rutile in eclogites provides Zr/Hf and Nb/Ta trends that indicate clinopyroxene fractionation. This is consistent with the occurrence of rutile in omphacite-rich parts of the rocks and supports their HP petrogenesis. In the felsic granulites rutile is associated with the amphibolite-facies garnet-bearing assemblage and its Nb/Ta and Zr/Hf primarily reflect fractionation by rutile. Zirconium-in-rutile temperatures are 800-835 °C for the felsic granulites and 860-895 °C for the eclogites. Titanium-in-zircon temperatures increase from ~735 °C (0.7-1.0 GPa) at 39.1 Ma to ~900 °C (>2.5 GPa) at 11.5 Ma; a further, abrupt increase toward 1000 °C at 11.1 Ma marks melting at the onset of eruption. The analytical uncertainty on the Miocene ages is small compared to the 28-Myr burial record, enabling precise dating of individual reaction and deformation events. These events are at least an order of magnitude shorter than the duration of burial, and evidently occurred in pulses recorded by the (re)crystallization of zircon or monazite. Reference: [1] Hacker et al. (2005) J Petrol 46 (8): 1661-1687.

Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

NASA Astrophysics Data System (ADS)

Silva, Luiz Carlos da; Pedrosa-Soares, Antonio Carlos; Armstrong, Richard; Pinto, Claiton Piva; Magalhães, Joana Tiago Reis; Pinheiro, Marco Aurélio Piacentini; Santos, Gabriella Galliac

2016-07-01

This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U-Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm-Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some of the oldest magmatic and metamorphic events ever found in the São Francisco craton basement, a ca. 3.4 Ga TTG generation and ca. 3.1 Ga metamorphism, as well as two younger orogenic events: the Rhyacian (2.15-2.05 Ga), accretionary event, correlated to the Mineiro belt, and the Ediacaran Brasiliano overprinting, event imposed by the Araçuaí orogen.

Mobilized plasma lead as an index of lead body burden and its relation to the heme-related indices.

PubMed

Sakai, T; Ushio, K; Ikeya, Y

1998-07-01

Plasma lead (Pb-P) from workers were distributed in two main fractions: a protein bound fraction and low molecular weight fractions. Lead mobilized into plasma by CaEDTA was mainly observed in the low molecular weight fraction corresponding to lead disodium ethylenediamine tetraacetic acid (PbEDTA). The peak levels of Pb-P was attained around 1.5 and 2.5 hours after the start of CaEDTA injection. Pb-P and blood lead levels (Pb-B) at 2 h after the injection were 4.26 (+/- 2.84) and 0.96 (+/- 0.27) fold of the initial levels just before the injection. Pb-P concentrations at 2 hours after the start of CaEDTA injection (MPb-P) were well correlated (r = 0.740) with amounts of lead excreted in urine for 24 h thereafter (MPb-U). log MPb-P as well as log MPb-U were correlated with Pb-B (r = 0.765 and 0.817, respectively). Correlation coefficients of lead body burden (MPb-P or MPb-U) vs the logarithms of the effect indices (delta-aminolevulinic acid (ALA) dehydratase, ALA in urine, coproporphyrin in urine, and erythrocyte zinc protoporphyrin) were higher than the correlation coefficients of exposure indices (Pb-B or Pb-U) vs the logarithms of the effect indices. Thus the biological effect monitoring is significant and reliable for evaluating the functional components of lead body burden (MPb-P or MPb-U).

Parity Nonconservation in Proton-water Scattering at 800 MeV

DOE R&D Accomplishments Database

Nagle, D. E.; Bowman, J. D.; Carlini, R.; Mischke, R. E.; Frauenfelder, H.; Harper, R. W.; Yuan, V.; McDonald, A. B.; Talaga, R.

1982-01-01

A search has been made for parity nonconservation in the scattering of 800 MeV polarized protons from an unpolarized water target. The result is for the longitudinal asymmetry, A{sub L} = +(6.6 +- 3.2) x 10{sup -7}. Control runs with Pb, using a thickness which gave equivalent beam broadening from Coulomb multiple scattering, but a factor of ten less nuclear interactions than the water target, gave A{sub L} = -(0.5 +- 6.0) x 10{sup -7}.

High-temperature metamorphism and its relation with magmatism (Svecofennian Belt, the Ladoga region, Russia)

NASA Astrophysics Data System (ADS)

Baltybaev, Shauket

2010-05-01

The Ladoga region, situated in the south-eastern part of the Fennoscandian shield, is subdivided into the Archean (ARD) and the Proterozoic (PRD) domains. The boundary between them is a wide shear-zone. The ARD consists mostly of AR-PR middle-low temperature gneisses and the PRD consists of turbidites, pelites, volcanics metamorphosed under HT-conditions (granulite facies). Metamorphism within the PRD is culminated at T= 800-900C and P=5-6 kbar. The peak of metamorphism of granulite facies is dated at 1881 Ma by Pb-Pb stepwise leaching method of rock-forming minerals of the granulites. Pb-Pb results are within error limits coeval with the U-Pb ages of metamorphic monazites. The same (1881Ma) age has gabbro-enderbites. Next stage of metamorphism lasts from 1881 to 1860 Ma under conditions of amphibolite facies. It was restricted with U-Pb, Pb-Pb, Sm-Nd data based on the closure temperature of zircon, monazite, garnet, sillimanite from gneisses, leucosomes of migmatites and synmetamorphic diorites and tonalites. The lowermost point of the trend shows P-T: ~3-4 kbar, 600C. By the time 1860 Ma K-rich granites were emplaced and the uppermost limit for granulite metamorphism comes from the ages of the aplitic/pegmatitic veins (1860-1850 Ma), which cut the K-rich granites. Thermal and tectonic settings can be described based on spatial and temporal changes during magma emplacement. The granulites of the PRD were produced by the emplacement of the extensive basic intrusion (gabbro-enderbites) into the lower-middle crust. A prolonged thermal flux over all area was supported by new generated dioritic and tonalitic melts, which were intruded into the middle crust. The final stage of tectono-metamorphic evolution was marked by emplacement of the K-rich granites. Numerical simulation of the process of magma emplacement (sequences: gabbro-enderbites, diorites and tonalites) and related heat production shows good correlation between intrusive activity and metamorphism of the surrounding rocks. Baltybaev Sh. K., Levchenkov O. A., Levsky L. K., Eklund O., Kilpeläinen T. 2006. Two metamorphic stages in the Svecofennian Domain: evidence from the isotopic geochronological study of the Ladoga and Sulkava metamorphic complexes. Petrology, 14(3), 247-261.

Isotopic chemical weathering behaviour of Pb derived from a high-Alpine Holocene lake-sediment record

NASA Astrophysics Data System (ADS)

Gutjahr, Marcus; Süfke, Finn; Gilli, Adrian; Anselmetti, Flavio; Glur, Lukas; Eisenhauer, Anton

2017-04-01

Several studies assessing the chemical weathering systematics of Pb isotopes provided evidence for the incongruent release of Pb from source rocks during early stages of chemical weathering, resulting in runoff compositions more radiogenic (higher) than the bulk source-rock composition [e.g. 1]. Deep NW Atlantic seawater Pb isotope records covering the last glacial-interglacial transition further support these findings. Clear excursions towards highly radiogenic Pb isotopic input in the deep NW Atlantic seen during the early Holocene, hence after the large-scale retreat of the Laurentide Ice Sheet in North America, are interpreted to be controlled by preferential release of radiogenic Pb from U- and Th-rich mineral phases during early stages of chemical weathering that are less resistant to chemical dissolution than other rock-forming mineral phases [2-4]. To date, however, no terrestrial Pb isotope record exists that could corroborate the evidence from deep marine sites for efficient late deglacial weathering and washout of radiogenic Pb. We present a high-resolution adsorbed Pb isotope record from a sediment core retrieved from Alpine Lake Grimsel (1908 m.a.s.l.) in Switzerland, consisting of 117 Pb compositions over the past 10 kyr. This high-Alpine study area is ideally located for incipient and prolonged chemical weathering studies. The method used to extract the adsorbed lake Pb isotope signal is identical to previous marine approaches targeting the authigenic Fe-Mn oxyhydroxides fraction within the lake sediments [5, 6]. The Pb isotope compositions are further accompanied by various elemental ratios derived from the same samples that equally trace climatic boundary conditions in the Grimsel Lake area. The Pb isotopic composition recorded in Lake Grimsel is remarkably constant throughout the majority of the Holocene until ˜2.5 ka BP, despite variable sediment composition and -age, and isotopically relatively close to the signature of the granitic source rock. In contrast, adsorbed Th and U concentrations (given in concentrations of ng/g of sediment) are indeed significantly elevated during the earliest part of the record, while other adsorbed metals such as Al and Ti display highest adsorbed concentrations during the mid-Holocene. Elements such as Nd display fairly constant normalised concentrations throughout the record. Hence, while our Pb isotopic record appears remarkably insensitive towards climatic perturbations seen during the Holocene, the various elemental records display a striking sensititivity towards the overall climate evolution of the Holocene. Finally, the rise and fall of the Roman Empire as well as the onset of the industrial revolution are clearly resolvable in our Pb isotopic records. References [1] Harlavan, Y. Y. et al. (1998), GCA 62, 33-46; [2] Gutjahr, M. et al. (2009), EPSL 286, 546-555; [3] Kurweil, F. et al. (2010), EPSL 299, 458-465; [4] Crocket, K.C. et al. (2012) QSR 38, 89-99; [5] Gutjahr et al. (2007) Chem. Geol. 242, 351-370; [6] Blaser, P. et al. (2016) Chem. Geol. 439, 189-204.

Small Volume Isotopic Analysis of Zircon Using LA-MC-ICP-MS U-Pb and Lu-Hf and Sub-ng Amounts of Hf in Solution

NASA Astrophysics Data System (ADS)

Bauer, A.; Horstwood, M. S.

2016-12-01

Crust-mantle evolution studies are greatly informed by zircon U-Pb and Lu-Hf isotopic datasets and the ease with which these data can now be acquired has seen their application become commonplace. In order to deconvolute geochemical change and interpret geologic variation in complexly zoned zircons, this information is most ideally obtained on the smallest volume of zircon by successive SIMS U-Pb and LA-MC-ICP-MS Lu-Hf isotopic analyses. However, due to variations in zircon growth zone geometry at depth, the Lu-Hf analysis may not relate to the lower volume U-Pb analysis, potentially causing inaccuracy of the resultant age-corrected Hf isotope signature. Laser ablation split-stream methods are applied to be certain that U-Pb and Lu-Hf data represent the same volume of zircon, however, the sampling volume remains relatively large at 40x30µm1. Coupled ID-TIMS U-Pb and solution MC-ICP-MS Lu-Hf work traditionally utilize whole-zircon dissolution ( 10-50ng Hf), which has the potential to homogenize different zones of geologic significance within an analysis. Conversely, modern ID-TIMS U-Pb methods utilize microsampling of zircon grains, often providing < 5ng Hf, thereby challenging conventional Lu-Hf acquisition protocols to achieve the required precision. In order to obtain usable precision on minimal zircon volumes, we developed laser ablation methods using successive 25um spot U-Pb and Lu-Hf ablation pits with a combined depth of 18um, and low-volume solution introduction methods without Hf-REE separation utilizing Hf amounts as low as 0.4ng, while retaining an uncertainty level of ca. 1 ɛHf for both methods. We investigated methods of Yb interference correction and the potential for matrix effects, with a particular focus on the accurate quantification of 176Lu/177Hf. These improvements reduce the minimum amount of material required for U-Pb and Hf isotopic analysis of zircon by about an order of magnitude. 1Ibanez-Mejia et al (2015). PreRes, 267, 285-310.

U-Pb isotope systematics and age of uranium mineralization, Midnite mine, Washington.

USGS Publications Warehouse

Ludwig, K. R.; Nash, J.T.; Naeser, C.W.

1981-01-01

Uranium ores at the Midnite mine, near Spokane, Washington, occur in phyllites and calcsilicates of the Proterozoic Togo Formation, near the margins of an anomalously uraniferous, porphyritic quartz monzonite of Late Cretaceous age. The present geometry of the ore zones is tabular, with the thickest zones above depressions in the pluton-country rock contact. Analyses of high-grade ores from the mine define a 207 Pb/ 204 Pb- 235 U/ 204 Pb isochron indicating an age of mineralization of 51.0 + or - 0.5 m.y. This age coincides with a time of regional volcanic activity (Sanpoil Volcanics), shallow intrusive activity, erosion, and faulting. U-Th-Pb isotopic ages of zircons from the porphyritic quartz monzonite in the mine indicate an age of about 75 m.y., hence the present orebodies were formed about 24 m.y. after its intrusion. The 51-m.y. time of mineralization probably represents a period of mobilization and redeposition of uranium by supergene ground waters, perhaps aided by mild heating and ground preparation and preserved by a capping of newly accumulated, impermeable volcanic rocks. It seems most likely that the initial concentration of uranium occurred about 75 m.y. ago, probably from relatively mild hydrothermal fluids in the contact-metamorphic aureole of the U-rich porphyritic quartz monzonite.Pitchblende, coffinitc, pyrite, marcasite, and hisingerite are the most common minerals in the uranium-bearing veinlets, with minor sphalerite and chalcopyrite. Coffinitc with associated marcasite is paragenetically later than pitchblende, though textural and isotopic evidence suggests no large difference in the times of pitchblende and colfinite formation.The U-Pb isotope systematics of total ores and of pitchblende-coffinite and pyrite-marcasite separates show that whereas open system behavior for U and Pb is essentially negligible for large (200-500 g) ore samples, Pb migration has occurred on a scale of 1 to 10 mm (out of pitchblende and coffinite and into pyrite and marcasite). Also, long-term continuous leakage of radioactive daughters of 238 U (probably 222 Rn) has occurred on scales of from approximately 100 mu m approximately 10 cm. The isotopic composition of unsupported radiogenic Pb in pyrite-marcasite seems to depend on the mineralogical microenvironment of the grains, so that the radiogenic Pb in pyrite-marcasite intimately intermixed with pitchblende-coffinite tends to be deficient in 206 Pb, and the radiogenic Pb in pyrite-marcasite in gangue tends to have excess 206 Pb. These systematics probably reflect differences between the average distances of Pb and 222 Rn diffusion since the formation of the ores.

High precision time calibration of the Permo-Triassic boundary mass extinction by U-Pb geochronology

NASA Astrophysics Data System (ADS)

Baresel, Björn; Bucher, Hugo; Brosse, Morgane; Schaltegger, Urs

2014-05-01

U-Pb dating using Chemical Abrasion, Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) is the analytical method of choice for geochronologists, who are seeking highest temporal resolution and a high degree of accuracy for single grains of zircon. The use of double-isotope tracer solutions, cross-calibrated and assessed in different EARTHTIME labs, coinciding with the reassessment of the uranium decay constants and further improvements in ion counting technology led to unprecedented precision better than 0.1% for single grain, and 0.05% for population ages, respectively. These analytical innovations now allow calibrating magmatic and biological timescales at resolution adequate for both groups of processes. To construct a revised and high resolution calibrated time scale for the Permian-Triassic boundary (PTB) we use (i) high-precision U-Pb zircon age determinations of a unique succession of volcanic ash beds interbedded with shallow to deep water fossiliferous sediments in the Nanpanjiang Basin (South China) combined with (ii) accurate quantitative biochronology based on ammonoids and conodonts and (iii) carbon isotope excursions across the PTB. Using these alignments allows (i) positioning the PTB in different depositional environments and (ii) solving age/stratigraphic contradictions generated by the index, water depth-controlled conodont Hindeodus parvus, whose diachronous first occurrences are arbitrarily used for placing the base of the Triassic. This new age framework provides the basis for a combined calibration of chemostratigraphic records with high-resolution biochronozones of the Late Permian and Early Triassic. Besides the general improvement of the radio-isotopic calibration of the PTB at the ±100 ka level, this will also lead to a better understanding of cause and effect relations involved in this mass extinction.

Igneous and tectonic evolution of the Batchawana Greenstone Belt, Superior Province: a U-Pb zircon and titanite study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Corfu, F.; Grunsky, E.C.

1987-01-01

U-Pb isotopic dating of zircon and titanite from all the major litho-tectonic units of the Batchawana belt, an Archean greenstone belt of the Abitibi Subprovince of the Superior Province in Canada, shows that the belt evolved during a period of about 60 Ma between about 2730 and 2670 Ma ago. Subsequent deformation of the supracrustal sequences produced isoclinal folding and culminated in metamorphism ranging from lower greenschist to amphibolite facies and anatexis related to the intrusion of syn- to late-tectonic plutons, four phases of which have ages of 2678 +4/-2 Ma, 2677 +/- 2 Ma, 2677 +/- 3 Ma, andmore » 2676 +/- 2 Ma. Two post-tectonic granitoid plutons in the center of the belt were intruded 2674 +/- 3 Ma and 2673 +/- 5 Ma ago and were followed by the emplacement of a composite mafic to felsic intrusion; a monzonite and a hornblendite from this intrusion yield identical ages of 2668 +/- 2 Ma. Titanite ages are identical or younger than the ages of coexisting zircons and reflect regional metamorphism and post-tectonic plutonism, but in a few cases they are younger and may record increased fluid activity along faults and the intrusion of mafic dikes. U-Pb zircon systematics, together with age and lithological relationships, suggests that the greenstone belt formed in an oceanic environment from material derived initially mainly from the mantle. Subsequent melting at the base of the thickening volcanic succession produced intermediate to felsic volcanic rocks, tonalites, and later granodioritic to granitic plutons leading to the final consolidation of the granite-greenstone terrain. 47 references.« less

From Permo-Triassic lithospheric thinning to Jurassic rifting at the Adriatic margin: Petrological and geochronological record in Valtournenche (Western Italian Alps)

NASA Astrophysics Data System (ADS)

Manzotti, Paola; Rubatto, Daniela; Darling, James; Zucali, Michele; Cenki-Tok, Bénédicte; Engi, Martin

2012-08-01

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic histories, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallisation at HT conditions; ages scatter from 263 to 294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Late Paleozoic-Early Mesozoic tectonic evolution of the Paleo-Asian Ocean: geochronological and geochemical evidence from granitoids in the northern margin of Alxa, Western China

NASA Astrophysics Data System (ADS)

Sha, Xin; Wang, Jinrong; Chen, Wanfeng; Liu, Zheng; Zhai, Xinwei; Ma, Jinlong; Wang, Shuhua

2018-03-01

The Paleo-Asian Ocean (Southern Mongolian Ocean) ophiolitic belts and massive granitoids are exposed in the Alxa block, in response to oceanic subduction processes. In this work, we report petrographic, geochemical, and zircon U-Pb age data of some granitoid intrusions from the northern Alxa. Zircon U-Pb dating for the quartz diorite, tonalite, monzogranite, and biotite granite yielded weighted mean 206Pb/238U ages of 302±9.2 Ma, 246.5±4.6 Ma, 235±4.4 Ma, and 229.5±5.6 Ma, respectively. The quartz diorites ( 302 Ma) exhibit geochemical similarities to adakites, likely derived from partial melting of the initially subducted Chaganchulu back-arc oceanic slab. The tonalites ( 246.5 Ma) display geochemical affinities of I-type granites. They were probably derived by fractional crystallization of the modified lithospheric mantle-derived basaltic magmas in a volcanic arc setting. The monzogranites ( 235 Ma) are characterized by low Al2O3, but high Y and Yb with notably negative Eu anomalies. In contrast, the biotite granites ( 229.5 Ma) show high Al2O3 but low Y and Yb with steep HREE patterns and the absence of negative Eu anomalies. Elemental data suggested that the biotite granites were likely derived from a thickened lower crust, but the monzogranites originated from a thin crust. Our data suggested that the initial subduction of the Chaganchulu oceanic slab towards the Alxa block occurred at 302 Ma. This subduction process continued to the Early Triassic ( 246 Ma) and the basin was finally closed before the Middle Triassic ( 235 Ma). Subsequently, the break-off of the subducted slab triggered asthenosphere upwelling (240-230 Ma).

Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-Turonian boundary interval, western interior Basin, USA

USGS Publications Warehouse

Meyers, S.R.; Siewert, S.E.; Singer, B.S.; Sageman, B.B.; Condon, D.J.; Obradovich, J.D.; Jicha, B.R.; Sawyer, D.A.

2012-01-01

We develop an intercalibrated astrochronologic and radioisotopic time scale for the Cenomanian-Turonian boundary (CTB) interval near the Global Stratotype Section and Point in Colorado, USA, where orbitally influenced rhythmic strata host bentonites that contain sanidine and zircon suitable for 40Ar/ 39Ar and U-Pb dating. Paired 40Ar/ 39Ar and U-Pb ages are determined from four bentonites that span the Vascoceras diartianum to Pseudaspidoceras flexuosum ammonite biozones, utilizing both newly collected material and legacy sanidine samples of J. Obradovich. Comparison of the 40Ar/ 39Ar and U-Pb results underscores the strengths and limitations of each system, and supports an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. The radioisotopic data and published astrochronology are employed to develop a new CTB time scale, using two statistical approaches: (1) a simple integration that yields a CTB age of 93.89 ?? 0.14 Ma (2??; total radioisotopic uncertainty), and (2) a Bayesian intercalibration that explicitly accounts for orbital time scale uncertainty, and yields a CTB age of 93.90 ?? 0.15 Ma (95% credible interval; total radioisotopic and orbital time scale uncertainty). Both approaches firmly anchor the floating orbital time scale, and the Bayesian technique yields astronomically recalibrated radioisotopic ages for individual bentonites, with analytical uncertainties at the permil level of resolution, and total uncertainties below 2???. Using our new results, the duration between the Cenomanian-Turonian and the Cretaceous-Paleogene boundaries is 27.94 ?? 0.16 Ma, with an uncertainty of less than one-half of a long eccentricity cycle. ?? 2012 Geological Society of America.

Evidence of early Archean crust in northwest Gondwana, from U-Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

NASA Astrophysics Data System (ADS)

Naidoo, Thanusha; Zimmermann, Udo; Vervoort, Jeff; Tait, Jenny

2018-03-01

The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U-Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U-Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565-633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636-996 Ma); and (2) smaller Mesoproterozoic (5%; 1004-1240 Ma), Palaeoproterozoic (11%; 1890-2476 Ma) and Archean (11%; 2519-3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3-1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial ɛHf values between -15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (ɛHf(i) +10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative ɛHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

U-Pb zircon geochronology of plutonism in the northern Peninsular Ranges batholith, southern California: Implications for the Late Cretaceous tectonic evolution of southern California

USGS Publications Warehouse

Premo, Wayne R.; Morton, Douglas M.; Wooden, Joseph L.; Fanning, C. Mark

2014-01-01

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W–trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206Pb/238U ages date the three older zones from west to east at 126–107 Ma, 107–98 Ma, and 98–91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith.The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma.The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith–type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith.

A re-evaluation of the Kumta Suture in western peninsular India and its extension into Madagascar

NASA Astrophysics Data System (ADS)

Armistead, Sheree E.; Collins, Alan S.; Payne, Justin L.; Foden, John D.; De Waele, Bert; Shaji, E.; Santosh, M.

2018-05-01

It has long been recognised that Madagascar was contiguous with India until the Late Cretaceous. However, the timing and nature of the amalgamation of these two regions remain highly contentious as is the location of Madagascar against India in Gondwana. Here we address these issues with new U-Pb and Lu-Hf zircon data from five metasedimentary samples from the Karwar Block of India and new Lu-Hf data from eight previously dated igneous rocks from central Madagascar and the Antongil-Masora domains of eastern Madagascar. New U-Pb data from Karwar-region detrital zircon grains yield two dominant age peaks at c. 3100 Ma and c. 2500 Ma. The c. 3100 Ma population has relatively juvenile εHf(t) values that trend toward an evolved signature at c. 2500 Ma. The c. 2500 Ma population shows a wide range of εHf(t) values reflecting mixing of an evolved source with a juvenile source at that time. These data, and the new Lu-Hf data from Madagascar, are compared with our new compilation of over 7000 U-Pb and 1000 Lu-Hf analyses from Madagascar and India. We have used multidimensional scaling to assess similarities in these data in a statistically robust way. We propose that the Karwar Block of western peninsular India is an extension of the western Dharwar Craton and not part of the Antananarivo Domain of Madagascar as has been suggested in some models. Based on εHf(t) signatures we also suggest that India (and the Antongil-Masora domains of Madagascar) were palaeogeographically isolated from central Madagascar (the Antananarivo Domain) during the Palaeoproterozoic. This supports a model where central Madagascar and India amalgamated during the Neoproterozoic along the Betsimisaraka Suture.

High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic evolution of the southern Superior Province

USGS Publications Warehouse

Schmitz, M.D.; Bowring, S.A.; Southwick, D.L.; Boerboom, Terrence; Wirth, K.R.

2006-01-01

High-precision U-Pb ages have been obtained for high-grade gneisses, late-kinematic to postkinematic granitic plutons, and a crosscutting mafic dike of the Archean Minnesota River Valley tectonic subprovince, at the southern ramparts of the Superior craton of North America. The antiquity of the Minnesota River Valley terranes is confirmed by a high-precision U-Pb zircon age of 3422 ?? 2 Ma for a tonalitic phase of the Morton Gneiss. Voluminous, late-kinematic monzogranites of the Benson (Ortonville granite) and Morton (Sacred Heart granite) blocks yield identical crystallization ages of 2603 ?? 1 Ma, illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the Minnesota River Valley terranes. Postkinematic, 2591 ?? 2 Ma syenogranites and aplitic dikes in both blocks effectively constrain the final penetrative deformation of the Minnesota River Valley subprovince. Monazite growth from 2609 to 2595 Ma in granulitic paragneisses of the Benson and Montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism. Neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the Mesoarchean continental Minnesota River Valley terranes to the southern margin of the Superior Province, along the western Great Lakes tectonic zone. Subsequent Paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion, expressed in the Minnesota River Valley by major WNW-trending tholeiitic diabase dikes dated at 2067 ?? 1 Ma, only slightly younger than the structurally and geochemically similar 2077 ?? 4 Ma Fort Frances (Kenora-Kabetogama) dike swarm of northern Minnesota and adjoining Canada. ?? 2006 Geological Society of America.

Lead isotope ratios in bone ash of blesbok (Damaliscus pygargus phillipsi): a means of screening for the accumulation of contaminants from uraniferous rocks.

PubMed

Nöthling, Johan O; Du Toit, Johannes S; Myburgh, Jan G

2014-09-19

This study was done to determine whether blesbok (Damaliscus pygargus phillipsi) from the Krugersdorp Game Reserve (KGR) in Gauteng Province, South Africa have higher concentrations of (238)U and higher (206)Pb/(204)Pb and (207)Pb/(204)Pb ratios in their bone ash than blesbok from a nearby control reserve that is not exposed to mine water and has no outcrops of uraniferous rocks. Eight blesbok females from the KGR and seven from the control site, all killed with a brain shot, were used. A Thermo X-series 2 quadrupole ICPMS was used to measure the concentrations of (238)U and lead and a Nu Instruments NuPlasma HR MC-ICP-MS to measure the lead isotope ratios in the tibial ash from each animal. KGR blesbok had higher mean concentrations of (238)U (P = 0.02) and ratios of (206)Pb/(204)Pb and (207)Pb/(204)Pb (P < 0.00001) than the control blesbok. The probability of rejecting the false null hypothesis of no difference in the (206)Pb/(204)Pb or (207)Pb/(204)Pb ratios between KGR and control reserve animals (the power of the test) was 0.999. The blesbok from the KGR accumulated contaminants from an uraniferous environment. The (206)Pb/(204)Pb and (207)Pb/(204)Pb ratios in tibial ash proved effective in confirming accumulation of contaminants from uraniferous rocks.

The Mesozoic Caosiyao giant porphyry Mo deposit in Inner Mongolia, North China and Paleo-Pacific subduction-related magmatism in the northern North China Craton

NASA Astrophysics Data System (ADS)

Wu, Huaying; Zhang, Lianchang; Pirajno, Franco; Shu, Qihai; Zhang, Min; Zhu, Mingtian; Xiang, Peng

2016-09-01

The Caosiyao giant porphyry Mo deposit is located in the Wulanchabu area of Inner Mongolia, within the northern North China Craton (NCC). It contains more than 2385 Mt of ore with an average grade of 0.075% Mo. In the Caosiyao mining district, Mo mineralization occurs mainly in a Mesozoic granite porphyry as disseminations and stockworks, with some Mo distributed in Archean metamorphic rocks and diabase as stockworks and veins. The host granite porphyry is composed of two different phases that can be distinguished based on mineral assemblages and textures: one phase contains large and abundant phenocrysts (coarse-grained), while the other phase is characterized by fewer and smaller phenocrysts (medium-grained). Zircon U-Pb-Hf analyses of the former phase yielded a concordant 206Pb/238U age of 149.8 ± 2.4 Ma with a 206Pb/238U weighted mean age of 149.9 ± 2.4 Ma and εHf(t) values ranging from -12.2 to 18.3, while the latter phase gave a concordant 206Pb/238U age of 149.0 ± 2.2 Ma with a 206Pb/238U weighted mean age of 149.0 ± 2.1 Ma and εHf(t) values ranging from -13.1 to 17.7. Five samples of disseminated molybdenite have a 187Re-187Os isochron age of 149.5 ± 5.3 Ma with a weighted average age of 149.0 ± 1.8 Ma, whereas six veinlet-type molybdenite samples have a well-constrained 187Re-187Os isochron age of 146.9 ± 3.1 Ma and a weighted average age of 146.5 ± 0.8 Ma. Thus, it is suggested that the Mo mineralization of the Caosiyao deposit occurred during the Late Jurassic (ca. 147-149 Ma), almost coeval with the emplacement of the host granite porphyry (ca. 149-150 Ma). The host granite porphyry is characterized by high silica (SiO2 = 71.52-74.10 wt%), relatively high levels of oxidation (Fe2O3/FeO = 0.32-0.94 wt%) and high alkali element concentrations (Na2O + K2O = 8.21-8.76 wt%). The host granite porphyry also shows enrichments in U and K, and depletion in Ba, Sr, P, Eu, and Ti, suggesting strong fractional crystallization of plagioclase, biotite, and accessory minerals. These observations, together with high SiO2 contents and a high differentiation index (DI = 89.04-92.44), indicate a strong differentiation of the granite magma. Based on geological, geochronological, isotope systematics, and geochemical studies, we propose, for the first time, a genetic model for the Caosiyao porphyry Mo deposit. Under a regional extensional setting caused by far-field tectonics related to the Paleo-Pacific subduction during the Late Jurassic, a series of geodynamic, magmatic, and ore-forming processes took place, including formation of multi-directional and multi-phase faults, emplacement of the granitic host rocks, and Mo mineralization. Highly silicic, highly oxidized, and alkali-rich granitic magma, derived from partial melting of old lower crust, intruded into the country rocks. This highly differentiated granitic magma and the exsolved ore-forming fluids, enriched in Mo, migrated upward and interacted with the wall rocks. Eventually, ore minerals precipitated in fractures, resulting in the extensive deposition of molybdenite.

Lead contamination and source in Shanghai in the past century using dated sediment cores from urban park lakes.

PubMed

Li, H B; Yu, S; Li, G L; Deng, H

2012-08-01

Lead contamination becomes of importance to urban resident health worldwide, especially for child health and growth. Undisturbed lake sediment cores are increasingly employed as a useful tool to backdate environmental contamination history. Five intact sediment cores collected from lakes in five urban parks were dated using (210)Pb and analyzed for total Pb content and isotope ratio to reconstruct the Pb contamination history over the last century in Shanghai, China. Total Pb content in the sediment cores increased by about 2- to 3-fold since 1900s. The profile of Pb flux in each sediment core revealed a remarkable increase of Pb contamination in Shanghai over the past century, especially in the latest three decades when China was experiencing a rapid economic and industrial development. Significant correlations were found between Pb fluxes in sediment cores and Pb emission from coal combustion in Shanghai. Coal combustion emission dominated anthropogenic Pb sources during the past century contributing from 52% to 69% of total Pb in cores, estimated by a three-end member model of Pb isotope ratios. Leaded gasoline emission generally contributed <30% of total Pb, which was banned by 1997 in the Shanghai region. Our results implicate that coal combustion-based energy consumption should be replaced, or at least partially replaced, to reduce health risks of Pb contamination in Shanghai. Copyright © 2012 Elsevier Ltd. All rights reserved.

Some Pb and Sr isotopic measurements on eclogites from the Roberts Victor mine, South Africa

USGS Publications Warehouse

Manton, W.I.; Tatsumoto, M.

1971-01-01

Five nodules of eclogite, one nodule of garnet peridotite and one sample of kimberlite from the Roberts Victor mine were analyzed for concentrations of U, Th, Pb, Rb and Sr and isotopic compositions of Pb and Sr. In the eclogites, U content ranges from 0.09 to 0.26 ppm, Th from 0.35 to 1.1 ppm, Pb from 0.79 to 5.5 ppm, Rb from 2.1 to 28 ppm and Sr from 133 to 346 ppm; 206Pb/204Pb ratios range from 14.8 to 18.5, 207Pb/204Pb from 14.9 to 15.7, 208Pb/204Pb from 35.2 to 38.5. The garnet peridotite contains 0.22 ppm U, 0.97 ppm Th, 1.05 ppm Pb, 6.9 ppm Rb and 108 ppm Sr and the kimberlite contains 2.5 ppm U, 30 ppm Th, 37 ppm Pb, 113 ppm Rb and 2040 ppm Sr. The lead in the eclogites has two components, a lead pyroextractable at 1100-1200?? and a non-pyroextractable residual lead. In three of the eclogites, which are to some extent altered, a proportion of the pyroextractable lead may be contaminating lead from the kimberlite, but an altered kyanite eclogite does not appear to be contaminated by this same kimberlite. The pyroextractable lead from a less altered eclogite contains a much larger proportion of 206Pb. Compositions calculated for the residual leads vary greatly. In many of the pyroextraction runs the primary eclogitic phases disappeared and the new phases plagioclase, clinopyroxene and a magnetic iron compound were formed. Why part of the lead should have been retained by these new phases is not understood. ?? 1971.

Permian-Carboniferous arc magmatism in southern Mexico: U-Pb dating, trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

NASA Astrophysics Data System (ADS)

Ortega-Obregón, C.; Solari, L.; Gómez-Tuena, A.; Elías-Herrera, M.; Ortega-Gutiérrez, F.; Macías-Romo, C.

2014-07-01

Undeformed felsic to mafic igneous rocks, dated by U-Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive ɛHf values (+3.8-+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) ( T DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative ɛHf values (-1 to -14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous-Permian magmatic arc that extended from southern North America to Central America.

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

NASA Astrophysics Data System (ADS)

Toteu, Sadrack Félix; Penaye, Joseph; Deloule, Etienne; Van Schmus, William Randall; Tchameni, Rigobert

2006-04-01

Ion microprobe U-Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700-665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100-950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa-Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west-central Africa.

Zircon crytallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

USGS Publications Warehouse

Bachman, O.; Charlier, B.L.A.; Lowenstern, J. B.

2007-01-01

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Miff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ??? 60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; Smith et al., 1996) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction). ?? 2007 Geological Society of America.

Zircon crystallization and recycling in the magma chamber of the rhyolitic Kos Plateau Tuff (Aegean arc)

USGS Publications Warehouse

Bachman, O.; Charlier, B.L.A.; Lowenstern, J. B.

2007-01-01

In contrast to most large-volume silicic magmas in continental arcs, which are thought to evolve as open systems with significant assimilation of preexisting crust, the Kos Plateau Tuff magma formed dominantly by crystal fractionation of mafic parents. Deposits from this ~60 km3 pyroclastic eruption (the largest known in the Aegean arc) lack xenocrystic zircons [secondary ion mass spectrometry (SIMS) U-Pb ages on zircon cores never older than 500 ka] and display Sr-Nd whole-rock isotopic ratios within the range of European mantle in an area with exposed Paleozoic and Tertiary continental crust; this evidence implies a nearly closed-system chemical differentiation. Consequently, the age range provided by zircon SIMS U-Th-Pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus. The age distribution from 160 ka (age of eruption by sanidine 40Ar/39Ar dating; Smith et al., 1996) to ca. 500 ka combined with textural characteristics (high crystal content, corrosion of most anhydrous phenocrysts, but stability of hydrous phases) suggest (1) a protracted residence in the crust as a crystal mush and (2) rejuvenation (reduced crystallization and even partial resorption of minerals) prior to eruption probably induced by new influx of heat (and volatiles). This extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned Aegean microplate acts as a refractory container for magmas in the dying Aegean subduction zone (continent-continent subduction).

Empirical evidence for stability of the 405-kiloyear Jupiter-Venus eccentricity cycle over hundreds of millions of years.

PubMed

Kent, Dennis V; Olsen, Paul E; Rasmussen, Cornelia; Lepre, Christopher; Mundil, Roland; Irmis, Randall B; Gehrels, George E; Giesler, Dominique; Geissman, John W; Parker, William G

2018-06-12

The Newark-Hartford astrochronostratigraphic polarity timescale (APTS) was developed using a theoretically constant 405-kiloyear eccentricity cycle linked to gravitational interactions with Jupiter-Venus as a tuning target and provides a major timing calibration for about 30 million years of Late Triassic and earliest Jurassic time. While the 405-ky cycle is both unimodal and the most metronomic of the major orbital cycles thought to pace Earth's climate in numerical solutions, there has been little empirical confirmation of that behavior, especially back before the limits of orbital solutions at about 50 million years before present. Moreover, the APTS is anchored only at its younger end by U-Pb zircon dates at 201.6 million years before present and could even be missing a number of 405-ky cycles. To test the validity of the dangling APTS and orbital periodicities, we recovered a diagnostic magnetic polarity sequence in the volcaniclastic-bearing Chinle Formation in a scientific drill core from Petrified Forest National Park (Arizona) that provides an unambiguous correlation to the APTS. New high precision U-Pb detrital zircon dates from the core are indistinguishable from ages predicted by the APTS back to 215 million years before present. The agreement shows that the APTS is continuous and supports a stable 405-kiloyear cycle well beyond theoretical solutions. The validated Newark-Hartford APTS can be used as a robust framework to help differentiate provinciality from global temporal patterns in the ecological rise of early dinosaurs in the Late Triassic, amongst other problems.

Determination of heavy metals in the fruit of date palm growing at different locations of Riyadh.

PubMed

Aldjain, Ibrahim M; Al-Whaibi, Mohamed H; Al-Showiman, Salim S; Siddiqui, Manzer H

2011-04-01

Exposure of heavy metals to human beings has risen dramatically in the last 50 years. In today's urban and industrial society, there is no escaping from exposure to toxic chemicals and heavy metals. Humans are more likely to be exposed to heavy metal contamination from the dust that adheres to edible plants than from bioaccumulation. This is because it is very difficult to wash off all the dust particles from the plant material before ingesting them. The objectives of this experiment were to determine the concentrations of lead (Pb) and cadmium (Cd) in washing residues and in the tissues of fruits of date palm growing in 14 sites of Riyadh and also to assess whether the fruits were safe for human consumption. The washing residues and tissue of date palm fruits collected from different sites showed the presence of significant amounts of the Pb and Cd. The concentration of Pb in the dust and fruit tissue increased with increasing anthropogenic sources. Therefore, fruits of date palm might be used as a pollution indicator; it might be recommend that fruits of date palm could be safe for human consumption after washing. The mean concentration of Pb and Cd in all the samples collected from different sites is within the safe limits recommended by FAO/WHO.

Protracted or multiple subduction of metapelites (Rhodope UHP domain, Greece)?

NASA Astrophysics Data System (ADS)

Krohe, A.; Wawrzenitz, N. H.; Mposkos, E.; Romer, R. L.

2012-12-01

The Rhodope domain formed along the suture between the European and the Apulian/Adriatic plate, which collided in the early Tertiary (closure of the Vardar/Axios ocean). Its metamorphic history includes UHP metamorphism documented by diamond inclusions in garnet (Mposkos & Kostopoulos 2001, Perraki et al. 2006, Schmidt et al. 2010), presumably of Jurassic age, and Eocene stages of MP and HP metamorphism. The age of UHPM is still a matter of debate: U-Pb SHRIMP ages extend from 184-172 Ma (monazite in metapelites) to ca. 42 Ma with clusters at 170-160, 150-140, 80-60, 50, 42 Ma, (U-Pb SHRIMP dating of zircon from amphibolitized eclogites and metapelites). These ages are interpreted to date subsequent stages of (U)HP metamorphism and decompression (Liati et al., 2005, Hoinkes et al. 2008, Bauer et al. 2006, Krenn et al 2010). However, these ages are obviously difficult to link with the metamorphic reactions. The metamorphic history has been interpreted in different ways, reflecting: (i) successive accretion of small terranes with rapid subduction and uplift histories (e.g. Liati et al. 2005); (ii) a composite of different tectonic units varying in earlier P-T histories, assembled by shear zones that reflect tectonic erosion and differential exhumation along the plate interface and that are now erased and overprinted (Krohe and Mposkos, 2002, Mposkos et al., 2010). These interpretations imply a different kinematics of the tectonic movements at depths, mechanical processes and process rates. Additionally, a protracted polymetamorphic history of larger volumes of the Rhodope UHP domain may be considered; e.g. the Kimi complex stayed in the lower crust for ca. 50-60 Ma after exhumation of the UHP rocks to this lower crustal level (Mposkos and Krohe, 2006). To constrain a precise age of the HP granulite facies and a minimum age of UHP metamorphism, we conduct an integrated structural, petrologic and geochronological study in a metapelite from the Sidronero Complex. The mineral assemblages Grt-Ky-Bt-Pl-Kfs-Qtz-Rt and Grt-Ky-Bt-Ms-Pl-Qtz-Rt, record a HP granulite facies metamorphism followed by upper amphibolite facies. The rock is particularly well suited for studying the granulite facies metamorphism, as it contains domains that are only weakly overprinted by later metamorphic episodes. ID-TIMS U-Pb ages of single monazite grains and fractions of few grains, that are only locally patchy-zoned and associated with garnet and kyanite, plot along the concordia between 64 to 60 Ma. One date of 55 Ma might represent Pb-loss during later fluid-induced dissolution-reprecipitation, probably related to biotite growth during the amphibolite facies overprint. On the base of these data, a model is discussed, in which rocks from the upper plate and HP-rocks that have been already exhumed, were dragged again into the subduction channel by subduction erosion Bauer et al. 2006, Lithos, 29, 207-228; Hoinkes et al. 2008, 3rd IGC Oslo, UHP-4; Krenn et al 2010, Tectonics, 29, TC4001; Krohe & Mposkos, 2002, Geol. Soc. Sp. Pub. 204, 151-178; Liati, A., 2005, Contrib. Mineral. Petrol., 150, 608-630; Mposkos, & Kostopoulos, 2001, EPSL, 192, 497-506; Mposkos & Krohe, 2006. Can. J. Earth Sci., 43, 1755-1776; Mposkos et al., 2010 Proc. XIX CBGA Congress, 100, 173-178; Perraki et al., 2006, EPSL, 241, 672-685; Schmidt et al., 2010, EJM, 22, 189-198.

U-Pb geochronology and paleomagnetism of the Neoproterozoic St Simeon dolerite dykes, Quebec: an eastern Laurentian perspective of Ediacaran Rodinia breakup

NASA Astrophysics Data System (ADS)

Pisarevsky, Sergei; Murphy, Brendan; Hamilton, Mike; Söderlund, Ulf; Hodych, Joseph

2013-04-01

The St Simeon (SS) mafic dykes (150 km NE of Quebec City) are now dated at 548 ± 1 Ma (U-Pb; baddeleyite). This age is similar to a published LA-ICPMS zircon age of 550 ± 7 Ma for the Mt. St-Anselme (MS) basalts, which supports previous inferences of (i) a genetic relationship between them, (ii) the pene-contemporaneity of OIB-type mafic magmatism in East Laurentia and (iii) the existence of two late Ediacaran plumes that attended the final breakup of Rodinia and opening of the Iapetus Ocean and Tornquist Sea. Both the SS dykes and the MS basalts were sampled for paleomagnetic study. The paleomagnetic pole for SS is similar to the previously published pole for coeval basalts (Skinner Cove, SC) from Newfoundland. Unlike SC, the St Simeon pole represents rocks which are unambiguously coherent tectonically with the Laurentian Craton. This new pole is also coeval with high quality poles from the Winter Coast (Baltica) and provides paleomagnetic constraints on the history of the final breakup of Rodinia and opening of Eastern Iapetus and Tornquist Sea.

Post-collisional magmatism in the Late Miocene Rodna-Bârgău district (East Carpathians, Romania): Geochemical constraints and petrogenetic models

NASA Astrophysics Data System (ADS)

Fedele, Lorenzo; Seghedi, Ioan; Chung, Sun-Lin; Laiena, Fabio; Lin, Te-Hsien; Morra, Vincenzo; Lustrino, Michele

2016-12-01

Post-collisional magmatism in the Late Miocene Rodna-Bârgău subvolcanic district (East Carpathians) gave rise to a wide variety of rock compositions, allowing recognition of four groups of calcalkaline rocks with distinctive petrography, mineral chemistry, whole-rock geochemistry and Sr-Nd-Hf isotope features. New U-Pb zircon datings, together with literature data, indicate that the emplacement of the four rock groups was basically contemporaneous in the 11.5-8 Ma time span. The low potassium group (LKG) includes the most abundant lithotypes of the area, ranging from basaltic andesite to dacite, characterized by K-poor tschermakitic amphibole, weak enrichment in LILE and LREE, relatively low 87Sr/86Sr, coupled with relatively high 143Nd/144Nd and 176Hf/177Hf. The high potassium group (HKG) includes amphibole-bearing microgabbro, amphibole andesite and amphibole- and biotite dacite, with K-richer magnesio-hastingsite to hastingsite amphibole, more marked enrichments in incompatible elements, higher 87Sr/86Sr and lower 143Nd/144Nd and 176Hf/177Hf. These two main rock groups seem to have originated from similar juxtaposed mantle sources, with the HKG possibly related to slightly more enriched domains (with higher H2O reflected by the higher modal amphibole) with respect to LKG (with higher plagioclase/amphibole ratios). The evolution of the two rock series involved also open-system processes, taking place mainly in the upper crust for the HKG, in the lower crust for LKG magmas. In addition, limited occurrences of generally younger strongly evolved peraluminous rhyolites and microgranites (Acid group) and sialic-dominated "leucocratic" andesites and dacites (LAD group) were also recognized to the opposite outermost areas of the district. These two latter rock groups were generated by the melting of a basic metamorphic crustal source (respectively in hydrous and anhydrous conditions), favored by the heat released by mantle melts from the adjoining central area. The peculiar distribution of the products of the four rock groups in well defined sectors argues for a strong control of the local crustal tectonic regime on magmatism, influenced by the change from a transpressional to trastensional stage.

Atmospheric dust contribution to budget of U-series nuclides in weathering profiles. The Mount Cameroon volcano

NASA Astrophysics Data System (ADS)

Pelt, E.; Chabaux, F. J.; Innocent, C.; Ghaleb, B.

2009-12-01

Analysis of U-series nuclides in weathering profiles is developed today for constraining time scale of soil and weathering profile formation (e.g., Chabaux et al., 2008). These studies require the understanding of U-series nuclides sources and fractionation in weathering systems. For most of these studies the impact of aeolian inputs on U-series nuclides in soils is usually neglected. Here, we propose to discuss such an assumption, i.e., to evaluate the impact of dust deposition on U-series nuclides in soils, by working on present and paleo-soils collected on the Mount Cameroon volcano. Recent Sr, Nd, Pb isotopic analyses performed on these samples have indeed documented significant inputs of Saharan dusts in these soils (Dia et al., 2006). We have therefore analyzed 238U-234U-230Th nuclides in the same samples. Comparison of U-Th isotopic data with Sr-Nd-Pb isotopic data indicates a significant impact of the dust input on the U and Th budget of the soils, around 10% for both U and Th. Using Sr-Nd-Pb isotopic data of Saharan dusts given by Dia et al. (2006) we estimate U-Th concentrations and U-Th isotope ratios of dusts compatible with U-Th data obtained on Saharan dusts collected in Barbados (Rydell H.S. and Prospero J.M., 1972). However, the variations of U/Th ratios along the weathering profiles cannot be explained by a simple mixing scenario between material from basalt and from the defined atmospheric dust pool. A secondary uranium migration associated with chemical weathering has affected the weathering profiles. Mass balance calculation suggests that U in soils from Mount Cameroon is affected at the same order of magnitude by both chemical migration and dust accretion. Nevertheless, the Mount Cameroon is a limit case were large dust inputs from continental crust of Sahara contaminate basaltic terrain from Mount Cameroon volcano. Therefore, this study suggests that in other contexts were dust inputs are lower, or the bedrocks more concentrated in U and Th, the dust contribution will not significantly influence U-series dating. Chabaux F., Bourdon B., Riotte J. (2008). U-series Geochemistry in weathering profiles, river waters and lakes. Radioactivity in the Environment, 13, 49-104. Dia A., Chauvel C., Bulourde M. and Gérard M. (2006). Eolian contribution to soils on Mount Cameroon: Isotopic and trace element records. Chem. Geol. 226, 232-252. Rydell H.S. and Prospero J.M. (1972). Uranium and thorium concentrations in wind-borne Saharan dust over the western equatorial north atlantic ocean. EPSL 14, 397-402.

U-series constraints on the Holocene human presence in the Cuatro Cienegas basin, Mexico

NASA Astrophysics Data System (ADS)

Noble, S. R.; Felstead, N.; Gonzalez, S.; Leng, M. J.; Metcalfe, S. E.; Patchett, P. J.

2010-12-01

U-series tufa ages dating a human trackway have been obtained, part of a larger Late Pleistocene - Recent palaeoclimate and human occupation study of the Cuatro Cienegas basin, NE Mexico. Our analytical approach, including tracer calibration, couples aspects of what we consider best practice in the U-series community with our U-Pb experiences which includes the EarthTime U-Pb tracer calibration exercise. The recently discovered trackway is near a small hydrothermal pool within the basin [1], an ecologically highly significant oasis in the Chihuahuan desert. The oasis comprises >200 freshwater hydrothermal pools and a river system, and the related ecosystem hosts >70 endemic species[2]. Pools are fed by waters that circulate a deep karstic system and that originate in the surrounding upper Jurassic-lower Cretaceous Sierra Madre Oriental mountains (>3000m) [3]. The area hosted nomadic hunter-gatherers during the Holocene, and possibly as early as Late Pleistocene (~12 ka BP). Despite the basin's ecological significance, only three palaeoenvironmental studies have been published to date, and limited geochronological constraints are available. A pollen study of drill core through peats and tufas proximal to the pools suggested a long period of climatic stability and biogeographic isolation[4], a notion supported by the large number of endemic species, but other palynological and plant macrofossil data suggest that large climatic changes occurred post Late Pleistocene [5]. The 10 m long in situ trackway is preserved in tufa and five samples from the uppermost surfaces were analysed to date the footprints. The tufas comprise clean carbonate with no petrographic evidence of replacement and little contaminant detrital material (on some exposed upper surfaces). Powdered tufa was processed following [6-8], and analysed by TIMS (Triton, U) and MC-ICP-MS (Th, Nu HR), although our future analyses will primarily be obtained on a Neptune. Samples were spiked with a 229Th/236U tracer calibrated against gravimetric solutions prepared from Ames high-purity Th metal crystal and CRM 112a U metal ingot rather than natural materials of assumed secular equilibrium. ICP-MS mass bias and Faraday-SEM gain was monitored using CRM 112a and an in-house 229Th-230Th-232Th solution. Most samples have relatively high U contents (~2 ppm U), moderate [230Th/232Th] = 29-44, and initial [234U/238U] ~ 1.92. We obtain an age of 7.24 ± 0.13 ka BP for this trackway based on an average of two samples of the uppermost tufa surface. Depth profiling of one sample shows consistently increasing age downwards (~370 a/cm). [1] Gonzalez, A.H.G. et al., 2006, Ichnos 16, 12-24;[2] Souza, V. et al., 2006, PNAS 103, 6565-6570; [3] Johannesson, K.H. et al., 2004, J.S.Am.Earth Sci. 17, 171-180; [4] Meyer, E. 1973, Ecology 54, 982-995; [5] Minckley, T.A. & Jackson, S. 2008. J. Biogeography 35, 188-190; [6] Edwards, R.L. et al., 1987, EPSL 81, 175-192; [8] Cheng, H. et al., 2000, Chem. Geol. 169, 17-33; [8] Potter, E.K. et al., 2005, EPSL 247, 10-17.

Isotope age of the rare metal pegmatite formation in the Kolmozero-Voron'ya greenstone belt (Kola region of the Fennoscandian shield): U-Pb (TIMS) microlite and tourmaline dating

NASA Astrophysics Data System (ADS)

Kudryashov, Nikolay; Lyalina, Ludmila; Mokrushin, Artem; Zozulya, Dmitry; Groshev, Nikolay; Steshenko, Ekaterina; Kunakkuzin, Evgeniy

2016-04-01

The Kolmozero-Voron'ya greenstone belt is located in the central suture zone, which separates the Murmansk block from the Central-Kola and the Keivy blocks. The belt is represented by volcano-sedimentary rocks of Archaean age of 2.9-2.5 Ga. Rare metal pegmatites (Li, Cs with accessory Nb, Ta, and Be) occur among amphibolite and gabbroid intrusions in the northwestern and southeastern parts of the belt. According to the Rb-Sr data, the age of pegmatites was considered to be 2.7 Ga. Until recently there was no generally accepted point of view on the origin of pegmatites. Now we have isotopic data for a range of rock complexes that could pretend to be parental granites for the rare metal pegmatites. These are granodiorites with the zircon age of 2733±Ma, and microcline and tourmaline granites, which Pb-Pb isochronal age on tourmaline from the tourmaline granite located near the deposit is estimated to be 2520±70 Ma. The pegmatite field of the Vasin Myl'k deposit with the lepidolite--albite--microcline--spodumene--pollucite association is located among amphibolites in the northwestern part of the belt. The deposit is represented by subparallel low-angle zoned veins up to 220 m long and 5 m thick dipping in the southeastern direction at an angle of 10° too 30°. The minerals of the columbite--tonalite group from Vasin Myl'k deposit include microlite, simpsonite, and torolite, and are the oldest among different minerals represented by several generations in pegmatites under consideration. Zircons from the pegmatites are mostly represented by crystals with the structure affected by the action of fluids that put certain restrictions on its use as a geochronometer of the crystallization process. Microlite from the pegmatite taken from the dump of a prospecting drill hole was used for U--Pb (TIMS). The mineral is represented by 0.5--1.0 mm long euhedral octahedral crystals. It is brown in color, and transparent. The microlite crystals were preliminarily cleaned from surface contamination being placed the ultrasonic bath with the 7N HNO3 solution. The discordia constructed for seven measured microlite weights is characterized by upper intercepts with concordia at 2454±8 Ma, which probably reflects the time of rare metal pegmatite crystallization, coeval with the age of tourmaline granites. Taking into consideration the obtained U--Pb age of microlite, it may be assumed that the tourmaline granites dated back to 2520±70 Ma served as the most probable parental rocks for rare metal pegmatites of the Vasin-Myl'k deposit. The work is supported by RFBR No 16-05-00367, No 16-05-00427 and project No 0231-2015-0005

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

USGS Publications Warehouse

Tosdal, R.M.

1996-01-01

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru?? from the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171??20 Ma and 1158??12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (???1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Bele??n Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher presentday 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

26Al- 26Mg and 207Pb- 206Pb systematics of Allende CAIs: Canonical solar initial 26Al/ 27Al ratio reinstated

NASA Astrophysics Data System (ADS)

Jacobsen, Benjamin; Yin, Qing-zhu; Moynier, Frederic; Amelin, Yuri; Krot, Alexander N.; Nagashima, Kazuhide; Hutcheon, Ian D.; Palme, Herbert

2008-07-01

The precise knowledge of the initial 26Al/ 27Al ratio [( 26Al/ 27Al) 0] is crucial if we are to use the very first solid objects formed in our Solar System, calcium-aluminum-rich inclusions (CAIs) as the "time zero" age-anchor and guide future work with other short-lived radio-chronometers in the early Solar System, as well as determining the inventory of heat budgets from radioactivities for early planetary differentiation. New high-precision multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) measurements of 27Al/ 24Mg ratios and Mg-isotopic compositions of nine whole-rock CAIs (six mineralogically characterized fragments and three micro-drilled inclusions) from the CV carbonaceous chondrite, Allende yield a well-defined 26Al- 26Mg fossil isochron with an ( 26Al/ 27Al) 0 of (5.23 ± 0.13) × 10 - 5 . Internal mineral isochrons obtained for three of these CAIs ( A44A, AJEF, and A43) are consistent with the whole-rock CAI isochron. The mineral isochron of AJEF with ( 26Al/ 27Al) 0 = (4.96 ± 0.25) × 10 - 5 , anchored to our precisely determined absolute 207Pb- 206Pb age of 4567.60 ± 0.36 Ma for the same mineral separates, reinstate the "canonical" ( 26Al/ 27Al) 0 of 5 × 10 - 5 for the early Solar System. The uncertainty in ( 26Al/ 27Al) 0 corresponds to a maximum time span of ± 20 Ka (thousand years), suggesting that the Allende CAI formation events were culminated within this time span. Although all Allende CAIs studied experienced multistage formation history, including melting and evaporation in the solar nebula and post-crystallization alteration likely on the asteroidal parent body, the 26Al- 26Mg and U-Pb-isotopic systematics of the mineral separates and bulk CAIs behaved largely as closed-system since their formation. Our data do not support the "supra-canonical" 26Al/ 27Al ratio of individual minerals or their mixtures in CV CAIs, suggesting that the supra-canonical 26Al/ 27Al ratio in the CV CAIs may have resulted from post-crystallization inter-mineral redistribution of Mg isotopes within an individual inclusion. This redistribution must be volumetrically minor in order to satisfy the mass balance of the precisely defined bulk CAI and bulk mineral data obtained by MC-ICP-MS. The radiogenic 208Pb*/ 206Pb* ratio obtained as a by-product from the Pb-Pb age dating is used to estimate time-integrated 232Th/ 238U ratio ( κ value) of CAIs. Limited κ variations among the minerals within a single CAI, contrasted by much larger variations among the bulk CAIs, suggest Th/U fractionation occurred prior to crystallization of igneous CAIs. If interpreted as primordial heterogeneity, the κ value can be used to calculate the mean age of the interstellar dust from which the CAIs condensed.

Long term accretion history (165-70 Ma) recorded by high-pressure rocks of Diego de Almagro Island (Patagonia, Chile): implications for understanding subduction zone interface tectonic processes

NASA Astrophysics Data System (ADS)

Angiboust, Samuel; Hyppolito, Thais; Glodny, Johannes; Cambeses, Aitor; Monié, Patrick; Garcia-Casco, Antonio; Calderon, Mauricio; Juliani, Caetano

2017-04-01

The Diego de Almagro Island preserves one of the rare remnants of the Mesozoic Chilean paleo-accretionary wedge. This complex, formed by MOR-basalts interleaved with metasedimentary rocks, comprises three major tectonic units with distinct P-T-t paths: the HP granulite (Lazaro unit), the garnet amphibolite (GA) and the blueschist (BS) units. HP granulite-facies metamorphic conditions in the Lazaro Unit are attested by Grt-Cpx-Zo-Prg assemblages associated with trondhjemitic leucosomes (c. 1.3 GPa, 750°C). U-Pb SHRIMP dating of zircon metamorphic rims yields a homogeneous age population of 162 ± 2 Ma for this HT event, in agreement with Sm-Nd dating of peritectic garnet (163 ± 2 Ma and 163 ± 18 Ma). In situ white mica Ar-Ar dating and multi-mineral Rb-Sr dating of LT mylonites (c. 450°C) along the base of the Lazaro Unit reveals partial resetting of HT assemblages during deformation between 115 and 72 Ma. GA unit rocks, structurally below the Lazaro unit, locally preserve eclogite facies parageneses (c. 570°C, 1.7 GPa) that underwent a pervasive stage of amphibolitization during decompression down to 1.3 GPa. U-Pb dating of zircon metamorphic rims and Rb-Sr dating indicate that amphibolitization in GA unit took place at 125-120 Ma. GA unit rocks have been also lately overprinted by another HP-LT assemblage as shown by Si-richer phengite rims and small blue amphibole overgrowths. Conversely, the underlying BS unit does not show strong amphibolite facies overprint as seen in GA and Lazaro units and exhibits slightly cooler peak metamorphic conditions (c. 520°C, 1.7 GPa). Rb-Sr and Ar-Ar dating of these blueschists yield deformation ages between 80 and 70 Ma, i.e. 50 Ma younger than the overlying rocks from the GA unit, and 90 Ma younger than Lazaro unit HP-granulites. This new report sheds light on the formation of the youngest and deepest HP rocks exposed along the Chilean subduction margin. The Diego de Almagro Island represents a unique window onto long-term tectonic processes rooted below the base of the accretionary wedge (c. 40-50 km). The exceptionally long residence time of the earlier accreted material -almost 100 Ma-, enables the record of multiple thermal gradient fluctuations and highlights the variability of the subduction interface thermal structure over tens of millions yrs.

SHRIMP-RG U-Pb ages of provenance and metamorphism from detrital zircon populations and Pb-Sr-Nd signatures of prebatholithic metasedimentary rocks at Searl Ridge, northern Peninsular Ranges batholith, southern California: Implications for their age, origin, and tectonic setting

USGS Publications Warehouse

Premo, Wayne R.; Morton, Douglas M.

2014-01-01

Twenty-four samples were collected from prebatholithic metasedimentary rocks along Searl Ridge, the north rim of the Diamond Valley Reservoir, Domenigoni Valley, centrally located in the northern Peninsular Ranges of southern California. These rocks exhibit progressive metamorphism from west to east across fundamental structural discontinuities now referred to as a “transition zone.” Documented structural and mineralogical changes occur across this metamorphic gradient. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb ages were obtained from detrital zircons from metasedimentary rocks through the transition zone. To the west, metapelitic and minor metasandstone units yielded numerous concordant 206Pb/238U ages between 210 and 240 Ma, and concordant 207Pb/206Pb ages at 1075–1125 Ma, 1375–1430 Ma, and 1615–1735 Ma, although distinct differences in provenance were noted between units. A few older 207Pb/206Pb ages obtained were ca. 2250 Ma and ca. 2800 Ma. Rocks of the eastern part of the transition zone include high-grade paragneisses that yielded numerous concordant 206Pb/238U ages between 103 and 123 Ma and between 200 and 255 Ma, and concordant 207Pb/206Pb ages at 1060–1150 Ma, 1375–1435 Ma, and 1595–1710 Ma. Some zircon results from these high-grade gneisses are marked by distinct Pb-loss discordia with lower-intercept ages of ca. 215 Ma and Paleoproterozoic upper-intercept ages. Younger ages between 100 and 105 Ma are mainly obtained from rims of some zircon grains that are characterized by low Th/U values (<0.1) and high U contents (>1000 ppm), indicating the likelihood of metamorphic zircon growth at that time. The similarity of zircon age populations between western and eastern units through the transition zone indicates that this fundamental structure probably dissects sediments of the same basin. This supposition is further supported by initial whole-rock Pb-Sr-Nd isotopic data that show similar average initial 206Pb/204Pb (18.65 to 18.9), 87Sr/86Sr (0.713 to 0.718), and εNd (−7 to −12) values for both the western and eastern units—values that also indicate the presence of significantly older crustal material in their provenance.Magmatic zircons from a diorite dike that crosscuts the foliation, but is itself subsequently metamorphosed, yielded a SHRIMP-RG concordia age of 103.3 ± 0.73 Ma, which is within agreement of an isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb age of 103.37 ± 0.25 Ma. A postmetamorphic, cross-cutting pegmatite yielded discordant U-Pb zircon age data, but euhedral, glassy monazite from the pegmatite yielded a slightly discordant 207Pb/235U age of 101.85 ± 0.35 Ma and a Th-Pb age of 97.53 ± 0.18 Ma, suggesting that this pegmatite was injected during or just after deformation ceased. The age and initial Pb-Sr-Nd signature for the dioritic dike indicate it was produced during the transition zone plutonism elsewhere in the northern Peninsular Ranges batholith, whereas the pegmatitic dike was derived from crustal anatexis.Collectively, these results indicate that this sequence of metasedimentary rocks was derived from mainly a Late Permian to Early Triassic igneous provenance that probably intruded Proterozoic crust. The sequence was subsequently metamorphosed during deformation of the Cretaceous continental margin at ca. 105 to 97 Ma.

U-Pb (zircon) and geochemical constraints on the age, origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu-Au district, southern Mongolia

USGS Publications Warehouse

Wainwright, A.J.; Tosdal, R.M.; Wooden, J.L.; Mazdab, F.K.; Friedman, R.M.

2011-01-01

Uranium-Pb (zircon) ages are linked with geochemical data for porphyry intrusions associated with giant porphyry Cu-Au systems at Oyu Tolgoi to place those rocks within the petrochemical framework of Devonian and Carboniferous rocks of southern Mongolia. In this part of the Gurvansayhan terrane within the Central Asian Orogenic Belt, the transition from Devonian tholeiitic marine rocks to unconformably overlying Carboniferous calc-alkaline subaerial to shallow marine volcanic rocks reflects volcanic arc thickening and maturation. Radiogenic Nd and Pb isotopic compositions (??Nd(t) range from +3.1 to +7.5 and 206Pb/204Pb values for feldspars range from 17.97 to 18.72), as well as low high-field strength element (HFSE) contents of most rocks (mafic rocks typically have <1.5% TiO2) are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc. The Late Devonian and Carboniferous felsic rocks are dominantly medium- to high-K calc-alkaline and characterized by a decrease in Sr/Y ratios through time, with the Carboniferous rocks being more felsic than those of Devonian age. Porphyry Cu-Au related intrusions were emplaced in the Late Devonian during the transition from tholeiitic to calc-alkaline arc magmatism. Uranium-Pb (zircon) geochronology indicates that the Late Devonian pre- to syn-mineral quartz monzodiorite intrusions associated with the porphyry Cu-Au deposits are ~372Ma, whereas granodiorite intrusions that post-date major shortening and are associated with less well-developed porphyry Cu-Au mineralization are ~366Ma. Trace element geochemistry of zircons in the Late Devonian intrusions associated with the porphyry Cu-Au systems contain distinct Th/U and Yb/Gd ratios, as well as Hf and Y concentrations that reflect mixing of magma of distinct compositions. These characteristics are missing in the unmineralized Carboniferous intrusions. High Sr/Y and evidence for magma mixing in syn- to late-mineral intrusions distinguish the Late Devonian rocks associated with giant Cu-Au deposits from younger magmatic suites in the district. ?? 2010 Elsevier B.V.

Re-evaluation of the origin and evolution of > 4.2 Ga zircons from the Jack Hills metasedimentary rocks

NASA Astrophysics Data System (ADS)

Nemchin, A. A.; Pidgeon, R. T.; Whitehouse, M. J.

2006-04-01

New data are presented on internal structures, U-Pb systematics and oxygen isotope compositions of eight detrital zircons with ages greater than 4.2 Ga, from the Jack Hills metasedimentary belt, Australia. Cathodoluminescence imaging, ion-microprobe U-Pb and oxygen isotope results show evidence for an extensive period of complex zircon growth, secondary reaction and U-Pb isotopic disturbance from 4.36 to 3.90 Ga. In addition many of the zircons have discordant U-Pb systems and excess common Pb indicating a superimposed, relatively recent, reaction between radiation damaged zircon and low temperature fluids. The significance of oxygen isotope compositions for zircons with complex internal structures and U-Pb systems is complicated by uncertainty in the origin of the grains and the unknown effect of later reactions. However, a minority of grains with sharp oscillatory zoning, uniform and concordant U-Pb systems, igneous Th-U ratios and low common Pb contents, are interpreted as undisturbed primary magmatic zircons. The oldest identified, oscillatory zoned, magmatic grain, with an age 4363 ± 20 Ma, is one of a few reported magmatic grains with this age, which is interpreted as the oldest reliable age for Hadean magmatic zircons. Mantle δ18O values are reported for these zircons. Younger oscillatory zoned zircon, including oscillatory zoned cores in complex grains, have δ18O values lower than 6.5‰, which are within the range of ion microprobe analysed δ18O values for zircons in high temperature equilibrium with the normal mantle rocks of 5.3 ± 0.6‰ (2 standard deviations). These values are also within the range of δ18O values found in lunar zircons. The absence of heavy oxygen in the grains that can be interpreted as primary magmatic zircons and the complex history over the period from 4.36 to 3.9 Ga, seen in all other Jack Hills zircons and reflected in the internal structures and U-Pb isotopic systems, questions the model for the early Earth involving long intervals of relatively temperate conditions from 4.4 to 4.0 Ga that were conducive to oceans and possibly life.

Arginase induction by sodium phenylbutyrate in mouse tissues and human cell lines.

PubMed

Kern, R M; Yang, Z; Kim, P S; Grody, W W; Iyer, R K; Cederbaum, S D

2007-01-01

Hyperargininemia is a urea cycle disorder caused by mutations in the gene for arginase I (AI) resulting in elevated blood arginine and ammonia levels. Sodium phenylacetate and a precursor, sodium phenylbutyrate (NaPB) have been used to lower ammonia, conjugating glutamine to produce phenylacetylglutamine which is excreted in urine. The elevated arginine levels induce the second arginase (AII) in patient kidney and kidney tissue culture. It has been shown that NaPB increases expression of some target genes and we tested its effect on arginase induction. Eight 9-week old male mice fed on chow containing 7.5 g NaPB/kg rodent chow and drank water with 10 g NaPB/L, and four control mice had a normal diet. After one week all mice were sacrificed. The arginase specific activities for control and NaPB mice, respectively, were 38.2 and 59.4 U/mg in liver, 0.33 and 0.42 U/mg in kidney, and 0.29 and 1.19 U/mg in brain. Immunoprecipitation of arginase in each tissue with AI and AII antibodies showed the activity induced by NaPB is mostly AI. AII may also be induced in kidney. AI accounts for the fourfold increased activity in brain. In some cell lines, NaPB increased arginase activity up to fivefold depending on dose (1-5 mM) and exposure time (2-5 days); control and NaPB activities, respectively, are: erythroleukemia, HEL, 0.06 and 0.31 U/mg, and K562, 0.46 and 1.74 U/mg; embryonic kidney, HEK293, 1.98 and 3.58 U/mg; breast adenocarcinoma, MDA-MB-468, 1.11 and 4.06 U/mg; and prostate adenocarcinoma, PC-3, 0.55 and 3.20 U/mg. In MDA-MB-468 and HEK most, but not all, of the induced activity is AI. These studies suggest that NaPB may induce AI when used to treat urea cycle disorders. It is relatively less useful in AI deficiency, although it could have some effect in those patients with missense mutations.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite {40Ar }/{39Ar } age constraints on tectonic evolution of Southwestern Gondwanaland

NASA Astrophysics Data System (ADS)

Mukasa, Samuel B.; Dalziel, Ian W. D.

1996-11-01

Zircon U-Pb and muscovite {40Ar }/{39Ar } isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic "back stop" of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the {40Ar }/{39Ar } age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Geology, zircon geochronology, and petrogenesis of Sabalan volcano (northwestern Iran)

NASA Astrophysics Data System (ADS)

Ghalamghash, J.; Mousavi, S. Z.; Hassanzadeh, J.; Schmitt, A. K.

2016-11-01

Sabalan Volcano (NW Iran) is an isolated voluminous (4821 m elevation; > 800 km2) composite volcano that is located within the Arabia-Eurasia collision zone. Its edifice was assembled by recurrent eruptions of trachyandesite and dacite magma falling into a relatively restricted compositional range (56-67% SiO2) with high-K calc-alkaline and adakitic trace element (Sr/Y) signatures. Previous K-Ar dating suggested protracted eruptive activity between 5.6 and 1.4 Ma, and a two stage evolution which resulted in the construction of the Paleo- and Neo-Sabalan edifices, respectively. The presence of a topographic moat surrounding Neo-Sabalan and volcanic breccias with locally intense hydrothermal alteration are indicative of intermittent caldera collapse of the central part of Paleo-Sabalan. Volcanic debris-flow and debris-avalanche deposits indicate earlier episodes of volcanic edifice collapse during the Paleo-Sabalan stage. In the Neo-Sabalan stage, three dacitic domes extruded to form the summits of Sabalan (Soltan, Heram, and Kasra). Ignimbrites and minor pumice fall-out deposits are exposed in strongly dissected drainages that in part have breached the caldera depression. Lavas and pyroclastic rocks are varyingly porphyritic with Paleo-Sabalan rocks being trachyandesites carrying abundant phenocrysts (plagioclase + amphibole + pyroxene + biotite). The Neo-Sabalan rocks are slightly more evolved and include dacitic compositions with phenocrysts of plagioclase + amphibole ± alkali-feldspar ± quartz. All Sabalan rock types share a common accessory assemblage (oxides + apatite + zircon). High spatial resolution and sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry yielded two clusters of zircon ages which range from 4.5 to 1.3 Ma and 545 to 149 ka, respectively (all ages are averages of multiple determinations per sample). U-Th zircon geochronology for selected Neo-Sabalan rocks agrees with the U-Pb ages, with the youngest zircon rims dating to ca. 110 ka. Because zircon crystallization predates eruption, this age represents the upper limit for the youngest eruptions of Sabalan. Valley-filling ignimbrites yielded variable U-Pb zircon ages which argue against these pyroclastic rocks being generated in a single caldera forming event. These results indicate that eruptions occurred more recently than previously indicated by K-Ar dating. Paleo-Sabalan and Neo-Sabalan volcanic rocks have similar geochemical characteristics, including enrichment of LILE and LREE relative to HFSE and HREE, respectively, and prominent negative Ti, Nb, and Ta anomalies. The trachyandesitic to dacitic rocks of Sabalan also share negative Eu anomalies. This, together with horizontal or slightly increasing Y vs. Rb trends, indicates fractionation of plagioclase-amphibole or plagioclase-clinopyroxene assemblages with negligible crustal assimilation (based on low and invariant Rb/Th). High degrees of mantle partial melting are inferred from high (La/Yb)N (from 28 to 48). Overall, the subduction-affinity of Sabalan volcanic rocks agrees with models of melt generation following a Quaternary slab break-off event coeval with continental collision.